hucklebarry: I like Thief 1 better. However, I have noticed that people tend to like what they play first better, especially in this franchise. There are also hundreds of fan missions available, so its worth the price of the game alone, but a really nice bonus as some of those fan missions are quite epic.
I played the series in order but prefer Thief II. The prevailing opinion seems to be that Thief II has superior level design and "Thiefy-ness", but Thief I has a better story and "atmosphere." However I disagree with this assessment, and always thought Thief II had a stronger social subtext than Thief I. Whereas the Dark Project was about Pagan gods run amuck and Garrett joining up with the Medieval Catholic church stand-in and going rogue (pun intended) to beat back the chaos, the Metal Age was more about dying religions, moribund gods, displaced faith, dispossessed people and this void being filled with the rise of cults, corrupt bureaucracies, and charismatic charlatans to the sterile hum of machines and a changing, industrialized City. I thought Thief II's plot was more of a slow-burn, none of the "GOTCHA!" moments or 180 turns of the first, but it was more menacing and had more sociological relevance than the fantasy genre tropes of the first game. I thought Karras was a more effective villain than Constantine, because by nature he was a rather nebbish, harmless-looking fellow with an absurd Elmur Fudd voice, and certainly not endowed with any great strength or supernatural power. And yet, like a steampunk Ron L. Hubbard, through clever political maneuvering and psychological manipulation he was able to become lord and master of an entire army of fanatics bent on ruling the brave new metal world (and those "servants"...chilling).
Mind you, I love Thief I too; I mean, there was nothing else remotely like it when it was released! But for the reasons I listed above and my own steampunk biases (Arcanum forever!) give the crown to Thief II. Many Thief series fans seem to have loathed the "robots" of The Metal Age but I found them fascinating.
If one doesn't care about any of that, from a gameplay perspective I think Thief II was superior with its various engine tweaks and additional do-dads. Also the mansion and human infiltration missions are much more sophisticated than counterparts in the first game, with more secrets and intricate layouts. Thief II may not have had a memorably schizophrenic "The Sword" mission to its name or a nail-biting "Boneyard", but it had fewer duds; most of the missions were consistently good. It had some unique ones too; everyone mentions "Life of the Party," (and it's great), but the oft-overlooked "Trail of Blood" was one of the most unique experiences I had in the Thief series, a true Alice-in-Wonderland mission, with Garrett in her shoes.
The only Thief II mission I hated playing was the final one in Karras's Cathedral; all the others I thought were very worthwhile. About half the missions in Thief I, however, I didn't particularly like for one reason or other (not necessary due to undead either). The Gold Missions were also a mixed bag; while "Song of Caverns" was superb, I thought "Mage Towers" the worst Thief mission of all time, with all that hideous platforming and all those droning mages in a very dull level layout. Not to mention those buggers needlessly, and I do mean *needlessly*, replaced the crabmen in Thief Gold's version of "The Lost City."
