There's no doubt Q3A runs faster than the UT2003 engine. UT2003 engine looks better, but not good enough to be considered jaw dropping when compared to the Q3A engine. But, both are about to be obsolete anyway, so who cares?
id software always manages to get good performance while pushing the envelope. That is why I expect DOOM3 to run better with full detail on than Half-Life 2. I could be wrong about this though (duh). DOOM3's lighting engine may suck video cards dry. I'd personally like Valve to release some benchmarks so we could see what to expect based on the computer we have... I'd especially like to see this if I had the minimum system reqs; 800MHz with a TnT2.
One thing HL2 will deffinately have over DOOM3 is their performance on slower machines. I suspect one of the reasons HL is such a huge multiplayer game is because it's low system reqs allow it to run on almost everyone's machine. I know several people who play CS that can't run W:ET or BF1942 without making several tweaks. So I suspect HL2 will dominate the multiplayer market. (Of course, another reason for this is that DOOM3 isn't going to have jack for multiplayer. But I'm looking forward to some 1 on 1 matches with it. DOOM deathmatch was always great because there were a lot of original hiding places, especially dark ones. I look forward to that style of deathmatch again). I'm sure Valve wants CS2 (Valve can't pretend this isn't in development) to be accesible to most fans of the original.
Tribes 3 (or whatever it is) better get cleaner. One problem I had with Tribes 2 was that it was overly complicated for no reason. You spawn in a crumby place, so you have to jog over 50 mountains to find a base so you can buy a decent gun. Then, you have to watch out for AI (turrets) while you attack.
BF1942 takes on more strategy than Tribes 2 with less complexity. It has resources (vehicle spawns) and player classes. Also, the vehicles drive much better.
Another thing Tribes 3 has going against it is bugs. The bugs in Tribes 2 were a joke. I've never seen a game patch itself to death before that one. Terrible product management and QA. It had a crumby interface, hard-to-navigate menus, the control wasn't as smooth as other FPS', and the vehicle controls were not intuitive.
Plus, the weapons sucked. I mean, who the hell wants a gun that shoots a spinning disk? In trying to be original, Tribes went with something stupid. Yes, we know every game has the Rocket Launcher, shotgun and sniper gun, but they do because those guns are the most fun. Spinning disks? Please.
On that note, I'd like to slap UT2K3 for not allowing rocket jumping. Yes, you can jump while you're in mid-air (which is retarded), but the high-flying acrobatics of Q2 and Q3A are non-existent. People used to be rewarded to mastering cool tricks, but in UT2K3 they're too accesible. This is the only thing that's kept me from buying UT2K3.