hercufles: Im new to the swat franchise, if your new wich one uis best to start with?
Before I begin - I'm not a fan of FPS games unless there is some sort of tactical or unusual element to them.
That means I enjoy SWAT3 and SWAT4, as wel as No One Lives Forever 1 & 2, things like Rainbox Six (old versions) and even things like Overwatch or Wolfenstein: Enemy Territory.
Quake, Doom, and so on? Not so much. In the SWAT games you won't be jumping around like a madman with a heavy .50 cal machine gun in one hand and a heavy grenade launcher in the other while still jumping up several feet into the air, dodging enemy fire mid-air sideways, reloading and then one-shotting them in their heads without a scope twenty trillion miles away - while still airborne and right through an armoured vehicle they happen to be hiding behind. Lag due to latency is also less of a thing compared to those traditional FPS.
Get ready for a jumble of words - or just read other people's opinions here, as I tend to agree with most of them.
I like both, but do prefer SWAT3 GOTY over SWAT4.
Why?
The controls were quirky but remappable, but the tactical co-op options and for instance the mechanism for throwing grenades is implemented much better in my opinion.
In SWAT4, you can toss a grenade either low or high (swing up or swing down), but in SWAT3 you could essentially control the force applied when throwing the grenade more subtly. You could throw it slow or fast in multiple levels by keeping the button pushed until released to throw.
The bots in SWAT3 weren't the brightest but the game shines in multiplayer co-op. And yet, the AI in SWAT3 followed tactical procedures in a more realistic manner than in SWAT4.
There is deatch match in there too, which I never cared about and there used to be tons of mods available on the Internet to tweak the game, with lots of interesting maps, behavioral realism tweaks like Stockholm Syndrome, other weapons other skins up to entirely new worlds.
The HUD overlay in terms of controls, speeding up (PgUp) / going dynamic and slowing down (PgDn) / going stealth... with the exception of the game not using AWSD as the defacto FPS layout all the controls just made sense to me (even the way you could give orders to team mates or bots). The controls however could be remapped anyway you like them.
SWAT4 I also enjoyed but basically just because of better graphics and less trigger happy AI (that has consequences though - read other people's comments to discover in what way).
The controls in terms of giving orders were slower and quirkier to me than SWAT3's. There should be no reason for it, but personally I never got used to them. SWAT4 has a completely different engine and plays more like a Rainbow Six or Counter Strike game than like SWAT3, which can be a good thing or a bad thing depending on your perspective and preference.
SWAT4 is a lot darker, has much better graphics and if you prefer something like simple team DM, is probably the way to go. However for DM there are much more enjoyable games available these days or even back then (games like LucasArts' Outlaws with the Kill the Fool with the Chicken game spring to mind, UT99 CTF as well).
When co-op, stealth and multiplayer mission-based ops and mission diversity (disarmb bombs, free hostages, arrest warrant, evaccuation, VIP detail) are your thing (including split up among two separate teams with separate entry points while fighting AI), then I can highly recommend SWAT3.
If you are just looking for a modernized version of SWAT3 which retains co-op and mission based focus but with a darker atmosphere and contemporary controls which plays slightly more like a traditional but tactially oriented FPS, SWAT4 is not a bad choice even if there may be better ones available by now.
One thing which is more realistic in SWAT4 compared to SWAT3 is the fact that the limited amount of weight you can carry around is more strict compared to SWAT3, meaning less grenades on one person etc. Almost everytyhing else, from a co-op point of view, I liked more in SWAT3 for some reason.
If you do opt for SWAT4 (also a great game), you may wish to have a look here:
https://www.moddb.com/mods/swat-elite-force There is also a SWAT4 remake mod which features HD graphics, but the Elite Force mod will bring more realism (and would thus have my preference).
The latter will address many of the grievances I experienced with SWAT4 and more resembles what SWAT4 probably could and should have been even if personally I still enjoy SWAT3 a bit more - that could also admittedly be due to nostalgia though.
In the end it comes down to what you prefer to be realistic. The gameplay or the graphics.
And even there, from a gameplay perspectice in some areas SWAT4 does actually offer an improvement over SWAT3 but in most areas SWAT3 is definitely much better.
So, from a "police tactictal procedure" perspective, I'd go with SWAT3 if you must choose.
If you can afford both, just get both - they won't disappoint those who enjoy mission-based tactical co-op FPS, even against AI.