I've never played Urban Terror that I can recall but play TF2 all the time. That said they look very different to me.
I think the biggest difference is that right now (according to the respective websites) Urban Terror has 890 users playing while TF2 has 42,467.
UT appears to be styled similar to Counter-Strike while TF2 has a very unique style. It is class based so each of the nine characters have different abilities and weapons.
TF2 has only been free for about a month and has pretty frequent content updates.
Just watch these "Meet the Player" videos to get an idea of the style and humor behind TF2 (sniper is my favorite vid):
http://wiki.teamfortress.com/wiki/Meet_the_Team
All the TF2 details you could want here:
http://wiki.teamfortress.com
But the best part is that they're both free so you can easily try them both out and decide which you like better!