Ir depends.
I really like UT 2004, and the music in the game a lot better.
UT 3 is very good, and many say it goes back to the roots of UT 99 which was the first Unreal Tournament.
But...Epic did a
They joined forces with Gamespy for the browser you use to search for games online in your interface.
Gamespy is notorious for going down, and when that happens, no servers will show, and also you can not log in to the game, as you need a free Gamespy ID which you create in the game.
Many have had a tournament going when Gamespy will go down, and they have to quit.
There is much hatred for Gamespy's marriage to the game spelled out over on the official UT board I use.
http://utforums.epicgames.com/forumdisplay.php?f=20
We did not have this problem with the previous UT games.
Other than that though, the game is great.
I have not played UT 2004 since picking up this game last November.
There are bonus packs out for it now from Epic and aftermarket, and custom map makers in the sections on the forum that have released classic redos of the older maps like Facing Worlds and many more.