The Weapons between UT and Q3A are different, vastly different. UT's weapons have two different fire modes. Q3A's rocket launcher fires one or lets you rocket jump. It's no longer reduces everything to gibs, now it's just a less powerful fast moving gun with some splash damage. Lastly UT comes with a second CD with mods and a level editor.
Some mods are chainsaw, insta-gib shock rifles, fat boy vs. slim, low gravity, and others.
Plus installing a mod is simple, choose it from the list and it's installed. No moving or backing up files or setting command line arguments. When people create mods it'll be easy to pop them in, also maps are downloaded to your PC before a match, but it does it pretty slow. Sorry Quake 3. Unreal is an all around fairer game with better maps, weapons, AI, game modes, and interface.