Here's my quick comparison of the 4 big Id-engine games out now for Mac:
Doom 3:
Really boring demo
Medium-length campaign
Almost no story
Very little variety of environments (all dark corridors)
Very little variety of combat (mostly things jumping out suddenly)
Almost no puzzles
Very difficult because things kill you with no possible warning until you re-play
Great scary atmosphere with cool demons
Very simple multiplayer, but some fun mods (like LMS co-op)
Quake 4:
Really brief demo (with no multiplayer)
Long campaign
Simple story
Nice variety of environments
Very good variety of combat (including vehicles)
Few puzzles
Difficult but in a less-frustrating way
Better lit, less atmospheric
More varied, fast-paced, Q3Arena-style multiplayer
Prey:
So-so demo
Short campaign (the biggest criticism)
A little more complex story (but pretty campy)
Great variety of environments, with wallwalking etc. (not just empty gimmicks) and some jawdropping effects in certain scenes
Very good variety of combat (only one vehicle but it's fun)
Great puzzles (but too few of them)
Easier than the other two (when you win you can unlock a tougher mode)
Interesting alien atmosphere--walls really feel alive
Multiplayer is a nice change, with wallwalking etc., but few players online
Quake Wars:
No demo yet
Best replay value--endless new tactics to experiment with
Very little story--you can play the campaign in any order, offline or online
Some variety of environments (12 very large maps, containing multiple areas of combat each)
Endless variety of combat (you can be Strogg or Human)
No puzzles, but some hidden strategic places to find
As easy or as hard as you want (online is tough--at first--but play as Medic and you'll do your team some good)
Very realistic feel, like you are fighting in real-world locations that weren't made up for a game
Multiplayer is what it's all about--although single-player bot allies/enemies are very good too
The first 3 are first-person linear "quests"--you progress from scene to scene with no choice in the matter until you reach the end.
If you like linear "quests" then I think Quake 4 and Prey are the best. I prefer Prey, but Quake 4 is a close second and I know why people say it's the best one. Doom offers scariness, which is fun, but not much else.
Quake Wars is an entirely different kind of game, focused on working together as a team, with each player choosing a specialty (healing, spying, bombing from above, etc.). You can play in endless different ways, adapting to what your enemy and teammates are doing. Each battlefield has one side defending, the other attacking, and different objectives to complete. QW is a prequel to Quake 4--it's about Strogg attacking Earth, rather than humans attacking Stroggos. See:
http://4newbies.planetwolfenstein.gamespy.com/ETQW/
All have excellent graphics. In the end, I'm not sorry I bought any of them.