I have both and while they are very different, Gears of War is just a mediocre game, graphics are horrible (it's UE3 so they are fine technically but everything is brown, gray or black and none of the locales are interesting), control is mediocre (aiming is the only thing that works out fine) and overall the game lacks charm and depth, it's all about mindlessly shooting locusts which isn't even fun because you have to shoot a locust a gazillion times before it even starts to bleed, there is no sense that your guns are something more than pea shooters with fancy sound effects. Plus I don't have to tell you it doesn't even run that well plus you get that horrible Microsoft Live gaming thing which makes it look like a console game, ugh, one of the worst purchases I've made this year.
Unreal Tournament 3 on the other hand is pretty fun, pretty similar to the past games but it just feels right, to be based in the same engine it's pretty different than GoW, the weapons feel AWESOME and the graphics are actually pretty colorful, fitting and interesting and technically better than GoW, well what can I tell you, it's good ol UT with some new paint and some refinements overall.
But that's just me, some people actually LOVED GoW and some people think UT3 is mediocre (apparently a lot of them since it isn't selling so well).