I'm setting up an character to play through BG1+ToSC, then BG2 and TOB. I plan to play the basic GOG versions, without any mods (except maybe something that will highlight objects with a hotkey in BG1; I haven't decided yet).

I enjoyed playing a Swashbuckler when I first played through BG2 many years ago. For this run, I'm debating between a Fighter/Thief and a straight Thief that will be kitted to Swashbuckler when I start BG2. I don't care about the loss of backstab since I tend to play the thief more like a fighter when in combat. Out of combat, my thief scouts and deals with traps.

I'll roll until I get 18 Str, 19 Dex, 17 Con (elf), and at least 10 in Int, Wis, Cha (I don't like going lower than 10 on sheer principle). There is a +1 str manual in BG1, so I'm not too concerned about 18/xx if I go Fighter/Thief. I'm currently planning to play an elf, but if a different race provides significant benefit then maybe I'll switch.

So, my question: Are there significant differences in fighting ability between the Fighter/Thief and the Swashbuckler? I'll probably forget to backstab, so loss of backstab is almost a non-issue to me. I like to use the character as a hiding scout first, then he'll be either front line or ranged as I see fit.

One massive difference: a F/T will progress much slower, which means your Swashbuckler will soon outpace a F/T in actual damage dealt, because of the bonuses to hit and damage they get every 5 levels, which mitigates not being able to take weapon mastery -- they *can* take 3 points in two weapon fighting however. Not only that, his/her armour bonuses will also soon outstrip heavy armour while using top leathers. They lose out on HP, but you asked about abilities not stats. Early on F/T wins. Late game Swashbuckler takes it.

If you want a ranged combat class that can also double as a scoutish character you should get an Archer and perhaps think about abusing the Short Bow of Gesen later in Baldur's Gate 2 to compensate a little for how ranged weaponry falls short in the end (the further you progress, the weaker it becomes - it is exceptionally powerful in BG1 but gets progressively worse in BG2).

The only reason to use a Swashbuckler is because you want a single class thief and still use dual wielding and pretending to be capable of going toe to toe with big boys. At the end of the game you will be a pseudo-F/T with horrendous THAC0 values, missing a full attack/round (which doubles into missing out on 2 attacks/round under Improved Haste) and low hitpoints. If you go single class thief, don't bother with more than 16 CON, because there's no hitpoint bonus beyond that for thieves (do take 18 CON if you go F/T, it is worth it).

Most damning is though, obviously, since your backstab multiplier is x1, that you cannot get Assassination, which is - if you don't *need* Whirlwind to perform 10 attacks/round - pretty ludicrous when you backstab-multiply your full spread of attacks, and as F/T you will actually be able to hit something. You think that damage bonus on a swashbuckler is nice? Try that to see fun numbers. Heh.

The AC bonus is, umh, nice, but really, once you reach a certain point in the second game and everything starts having THAC0s in the lowish negative numbers (in the first, you'll always have the advantage as fighter anyway, due to the low level cap) and throws high level spells at you like there's no tomorrow you'll be glad to have them extra hitpoints from those fighter levels. ;)

The fighter/thief will be vastly superior for about 90 percent of the game, the swashbuckler slightly when the game is always over.

The specialized fighter/thief will start with 2.5 attacks per round for 5-10 damage each with a non-magical composite long bow, the thief will have 2 attacks per round for 1-6 damage each if you also consider the better hit chance the fighter/thief will do about 3 times as much damage.
The swashbuckler won't get even close before taking Whirlwind HLAs in ToB.
The multiclass will also be less vulnerable because of more HP and critical hit protection and other bonuses by helmets.

The pure class thief levels faster so NPCs you meet in BG2 will be higher level (in the very beginning of BG1 it will be the opposite) and areas with level scaling might have stronger monsters.

Swashbucklers using the thief attack & THAC0 table are a real deal breaker for the class, even if you waste the Gauntlets of Extraordinary Specialization on them once they can use it, they'll just forever trail behind in attacks/round and THAC0. To be fair, there's an alternative for Belm or Kundane (which are, im my opinion, an absolute must have for any serious dual wielders) for thieves who pick the Use Any Item HLA - the Scarlet Ninja-To from the Collector's Edition merchants in the Copper Coronet.

That one has an enchantment of +3 and can do some poison damage on a failed save but is limited to the Monk class who usually don't any weapons at all - but UAI can circumvent that limitation, and +3 is a good enough enchantment for almost any battle, even Demogorgon, but I wouldn't really tank that particular fight with a swashbuckler (or any single class thief or thief kit for that matter).

In general that's barely a problem - sure there are some things you won't be able to hit with your +2 or +3 enchanted off-hand but the extra attack (extra two attacks under improved haste) more than make up for that. It even beats Crom Fayr's 25 STR buff, unless you're dual wielding on a class that can't use swords where you simply don't have that option and need to make due with what you can use.

If you play a swashbuckler you'll mainly rely on Whirlwinds near the end, extra attacks because of off-hand won't be very important any more.

Yeah, that's true, i just sort of don't rate Swashbucklers as "serious" dual wielders and got carried away a little. :D

Just to see some numbers, I created a new swashbuckler (S) and a new fighter/thief (F/T) at the start of SoA, a new pair at the start of ToB, then I set the ToB XP for both to 5mil (the amount of XP that seems to be the target for end of ToB). The characters have no equipment and their stats are 18/00, 19, 17, 18, 18, 18 (str, dex, con, int, wis, cha). Further boosts to CON would give the F/T a little more hp, while other stats affect both mostly equally. If you see something else I should consider, please advise me.

I didn't compare BG1, since it would be T vs F/T (and the end game overlaps somewhat with start of SoA).

Start SoA, 89000 EXP
Level: S: 8, F/T: 6/7
Base THAC0, Real THAC0: S: 17, 13; F/T: 15, 12
AC: S: 4, F/T: 6
Thief Skills: S: 365, F/T: 340
HP: S: 70, F/T: 78
Saves: S: 12/12/11/15/13, F/T: 11/12/11/13/13
Prof Points: S: 4, F/T: 6
Attacks per round: S: 1, F/T: 1 (1.5 with weapon prof)
Backstab: S: 1x, F/T: 3x

Start ToB, 2.5 mil EXP
Level: S: 21, F/T: 12/15
Base THAC0, Real THAC0: S: 11, 4; F/T: 8, 5
AC: S: 1, F/T: 6
Thief Skills: S: 690, F/T: 540
HP: S: 108, F/T: 121
Saves: S: 9/6/8/12/7, F/T: 5/7/6/5/8
Prof Points: S: 7, F/T: 8
Attacks per round: S: 1, F/t: 2 (2.5 with weapon prof)
Backstab: S: 1x, F/T: 5x

End ToB?, 5 mil EXP
Level: S: 32, F/T: 18/21
Base THAC0, Real THAC0: S: 10, 1; F/T: 3, 0
AC: S: -1, F/T: 6
Thief Skills: S: 965, F/T: 690
HP: S: 130, F/T: 132
Saves: S: 9/6/8/12/7, F/T: 3/5/4/4/6
Prof Points: S: 10, F/T: 10
Attacks per round: S: 1, F/t: 2 (2.5 with weapon prof)
Backstab: S: 1x, F/T: 5x
HLA: S: 9 picks total (all Thief); F/T: 10 picks total (Fighter and Thief)

Over the course of the game, this seems to be the result:
THAC0: About even, with F/T usually being a point or two ahead.
AC: S is ahead, and slowly advances his lead with more levels. Is there a hard cap on max (best) AC you can have?
Thief Skills: Starts close in SoA, but then S pulls ahead and can cover everything by himself (including pick pockets and illusions) by the start of ToB. F/T sort of catches up towards the end of ToB, since after a certain point higher skills just don't mean much.
HP: F/T starts pretty far ahead, widens the lead, then S has started catching up by the end of the game.
Saves: S hits a cap on base saves before ToB, while F/T widens the gap as his fighter saves start coming online (much later). F/T leads by a lot here, especially against breath weapons.
Prof Points: F/T has more options, but S catches up. Either character still only has 2 hands though.
Attacks per round: F/T pulls ahead here. Whirlwind affects both equally, though.
Backstab: Obviously the F/T wins here. However, using this conflicts some with the better Attacks per round, unless you just lead a fight off with it.
HLA: F/T has 1 more pick at the end, but possibly is forced to split them between his classes (I don't remember). If it is possible to mix Whirlwind with Assassination, that would be neat. However, nothing can compete with multiple spike traps for making things dead.

TL;DR: Overall, it looks like the Swashbuckler is a thief with fighting ability, while the Fighter/Thief is a fighter with thieving ability.

[Edit: Slight change to my TL;DR analysis.]

Fighter thief sounds great. Especially dualled. Because you can pick up a spicy kit. Berserker thief, or Kensai thief. They are both nasty pieces of work! You don't really need much more fighter levels than 13, at which you get your second fighter melee attack per round! And you have enough xp to hit max thief level, minus 1. Also, you get enough remaining points to max pickpocket, hide in shadows and the other skill which together affect your overall hiding/stealth, and have enough for raising all other abilities to 100 and a little higher.

Being a swashbuckler is not as effective as having fighter levels thrown in, yet it is still a very close substitute. Its penalty in thief points is a bit troubling, but i trust that with potions and other in game tricks, it will not give you any trouble. Same goes for multi char, because eventually looses quite a number of thief levels, ergo skill points to allocate.

A fighter/thief is going to have much more hitpoints, backstab with greater ease (better THAC0) and effectiveness in general (swashbucklers loose on backstab, too), have enough points to utilize all thief abilities to their maximum potential without accessories such as pots, and endgame sacrifice fighter HLAs for more thief HLA points (picking up more special traps never hurts). And don't forget the kits, are overkill. Despite the fact you loose on resistances other races have passive, and the rolls.

If you are hellbent on making a standalone thief in 1, though, and change him/her into whatever you want in 2, keep in mind, that in 1's premade characters, you can import a certain halfling, with ERRONEOUSLY huge points into thieving skills, 19 dexterity, and 16 constitution. Also, Halflings are ideal thieves throughout the BG saga, and not only from RP perspective, but for racial bonuses, as well... And they perform adequately on this role, actually, too good for you not to notice. Importing it to 2 results into reallocating the thief skill points, and then it becomes a normal char, loosing the insanely high thief skills having throughout all of 1; yet, it is still a halfling, with great stats, and can be a perfect thief in 2, especially with a kit thrown in.