UT is slower paced then Quake. That's one big difference. I remember trying the UT Demo after the Q3 demo and found UT to be sluggish. But the lousy(imo) RL in Q3 kept me from going back and I'm glad
UT weapons have two firing modes, some of them have more.
Eg the RL, you can fire single rockets, multiple upto six, or launch grenades. If that's not enuff you can fire multiple rockets in two different formations, and oh yeah it also has a homing feature if you can stay on your target long enough.
Another big difference is the inclusion of team games in UT-tho Q3 team arena was released to play catch up-but you'll find assault, team DM, CTF, Domination (standard version-GOTY Edition has more mods) out of the box. So more team oriented stuff...tho stay away from assault online(imo).
Cosmetically UT is supposed to have weaker graphics. It can't do curved surfaces but if they are'nt as good, the difference isn't that much imo. You can check out Screen shots and make up your own mind.
I'm not sure how the cheating online issue is in Q-scene but in UT it is contained. It does exist, but it's contained thx to some cool members of our community.
I would highly recommend you get the Game of The Year Edition as you won't have to DL anymore pathces and stuff to play, plus you get 3 extra mods which are pretty damn good.
If you have high speed Inet access try the demo like gnome said.
OH yeah UT is processor intensive so hopefully you have something more then a 400 celeron/k6 -2. Anything less I would get the demo first.
Hope I covered stuff that helps you out.
Edit:Ach Sele beat me by a minute....why ya lousy ....