I left Half-Life at v1.0.0.1 and couldn't decide whether or not to buy UT or Q3A. After seeing the number of cheats available for Q3A, I decided on UT.
UT has great graphics, sound, AI, etc... We likely all agree on that (this being PlanetUnreal and all). What really makes UT stand apart from Q3A (correct me if I'm wrong; I don't know if Q3A can do the same) is it's inclusion of Unreal 1 weapons and monsters which enables it to play the old Unreal 1 maps. It's like having two games in one (minus the single player experience of Unreal 1).
I've only played the Q3A alpha test and I didn't have much fun with it. I prefer team oriented modes of play, (particularly with different classes) not mindless gibbing; there needs to be an objective for me to enjoy the game. Anyway, this is where Q3A has the advantage over UT. Sure UT has TP, CTF, DOM, and AS, but Q3A has Q3F - a revision of Quake TF, the ultimate team mod (IMO).
I don't recall which came out first, UT or Q3A, but it seems to me that the UT mod teams are slow in developing QUALITY mods. I'm talking about mods now, not mutators. Then there are the teams that are developing the same thing such as Infiltration, Tactical Ops and Strike Force. They're all the same Counter-Strike for UT wanna-be's! Why not combine their efforts to make a better mod faster? I guess that's up to them though.
Sorry for rambling. My point is this. UT is better, but Q3A has the edge on quality mods; which in the end, is what keeps a game alive.