Everyone keeps moving back and forth between arguing over gameplay, which is somewhat engine specific, but mostly determined by the company that develops the game, and engine eliteness.
The UT engine can handle big outdoor areas.
The Q3TA "engine" can sort of handle big outdoor areas.
UT bots are only as smart as the mapper makes them.
Q3A bots are only as smart as luck during the compile makes them.
I used to play UT a LOT and was anti-Q3 for a very long time, until I got a decent net connection. After playing Q3 for a few years now and trying to go back to UT, I've got my own opinion formed. UT seems more like a spamfest where anyone is capable of killing anyone, regarldess of skill, and the characters feel like paper when jumping around or getting hit. Q3 can be a spamfest if the map encourages it, but not to the same level as can be found in UT (multiple rockets at once, anyone?). The players in Q3 feel much heavier, like they're not a piece of styrofoam running around, which, in my opinion, makes the game feel more sturdy.
#40 - The editor that came with Unreal Tournament was ass. It would crash over the littlest things and many times you'd have walls that would disappear.
#54 - UT bot AI quality depends on the mapper. When someone makes a map, they have to place paths for the bots to follow, and tell them when to impact jump and similar. Q3A bots move around on their own (after compiling a bot script) but their difficulty setting really only seems to change how accurate their shots are. On the easiest setting, you can stand still and the bot will have trouble hitting you, where on the hardest setting, you'll always be railed no matter where you are. They're both pretty good, but are far from perfect.
Doom3 and UT2k3 both look pretty awesome and I plan on buying them both.