I just started playing Thief II The Metal Age right after finishing Thief Gold. I have not progressed much, still in early missions but so far I'd say I like Thief II better.

I like both, but i definitively found THIEF1 much better in it´s plot, and also in the gameplay. THIEF2 has better controls, but the levels in my opinion were just too big.

To me, the Thief series has been a lot like Assassin's Creed: The first games in both series felt like mere trial runs, while the second games both blew my mind.

I don't know, I think this comparison is unfair to both Thief and Assassins Creed.
The gameplay of the second AC is definitively much much better than that of the first, but I find the story really annoying and overdone (and there's some parts It could really do without (Cart mission, Flying mission, annoying but easy platforming sequences, that only get hard when the bad controls interfere (Assassins Tombs))). In many ways the AC series tries to outdo itself with added BS every time and would be much better if it just stuck to pseudostealth, parcour and stabbing people. I mean this city management was kinda fun and hiring thievess and all, but it feels kinda superfluous and I would have preferred a slightly leaner game without all that crap.

AC 1 feels very very unfinished in many ways, but I find I still slightly prefer it to the second for above reasons. Also AC has a kinda gray-ish story, where everyone you assassinate seems to be "kinda not that evil after all and maybe slightly right" and some people you work with/for a major dicks, whereas in 2+ all the Templars are genocidal arseholes and all Assassins saints.

Thief Gold on the other hand feels pretty polished and has so much atmosphere, incredible mission design (assassins, the sword...) and really good sound design (that IMHO outdoes most contemporary games easily. There's not many games where you can hear exactly where threats are before you can see them).
The parts of Thief 2 I've played so far feel even better, but even Thief Gold easily trumps most games I know and considering it a mere trial is kinda unfair.

TL;DR AC2 and AC both have flaws (AC1 atory great, gameplay flawed; 2 gameplay much better but shit story and lots of added annoying things); Thief Gold and Thief 2 extremely rock (although I slightly prefer 2). Comparing the series is not really fair on all of these games.

That's pretty much the impression I have. Much less so with Thief, though, as it's mostly very polished. But as great as it is, the first Thief has a really big flaw: The overuse of zombies and buricks. It's such a boneheaded decision, that I really have to wonder how they could ever consider it a good thing. In fact, I'm tempted to skip the game and just continue with Thief 2, since I'm sick of this crap.

How different is Thief from TG?Is the old version of the Lost City available anywhere?

@Myrokratios: I totally agree with the burricks and zombies. I find them really annoying. Especially when they appear in nearly every single level and are hard to avoid (especially burricks. they seem to be everywhere.) I'm not so keen of the small annoying spiders either.

Apart from the additional levels (Thieves guild (liked it at first, but it was too big), Mage Tower (annoying) & Songs of the Caverns (odd storywise, but the level was fun)), the changes to Lost City & the updated graphics, it is the same game. And I sadly don't know of any mods that changes Lost City back to its original state.

Burricks can actually be treated like any other living enemy, they follow the same rules as humans (sneak up behind them & hit them with a blackjack, and they will go down, and they seem to be as good at detecting sound as any other humanoid creature). Once I realized that, I started to enjoy them. Plus, they are kinda cute.

Thieves Guild was fun at first, but by the end I was just running around and blackjacking people because I got fed up with trying to navigate the maze elegantly. They overdid the maze a bit I'd say. Emptying the manors of the bosses was fun though.

Didn't know Mages' Tower was Gold as well. It took a while until I noticed I would have to go through each f... tower. Whoever designed the level must love platformers. It's especially annoying when mages get stuck and end up patrolling in patterns where it's near impossible to not alarm them (and every surface seems to be metal or marble).

I hope talk better about this topic soon, as im almost finished again THIEF 1 and I will start then with THIEF2.

I´m still loving the first one. Even playing it now.

Thief II, its just better everyway, more varied missions, bigger areas etc..

I hope 4 catches the magic of Thief II.
& has this amazing song in it:
http://www.youtube.com/watch?v=5C9kjLmtQMM

Not having played the original Thief (only TG,) how was the original version of Lost City better?
Oh, and I prefer Thief 2. Much more thievery, much less monster hunting. More enjoyable story, and the extra industrialization caused by the rise of the Mechanists made for a cooler setting, too. Heck, even just straight exploration is a joy, and I sometimes spend waaaay more time than a mission needs, just poking around mansion rooms and enjoying the ambient sounds, like the upstairs of Rumford Manor in the first mission...

If I'm not mistaken, the Gold version of the Lost City is populated by Mages, making it more of a human vs human mission

I prefer Thief 2, honestly. The missions are georgeous, and more fun.

Ya, I seem to recall the original version of Lost City was filled with more mantises, and burricks, etc.

They replaced most of them with mages.