Have you considered Oblivion?
I'm looking for a shooter when I get my Xbox360. I haven't played a good satisfying game for a looooong time, especially a good shooter (I don't own an xbox).
I know I'll have a great time with Perfect Dark: Zero or Quake 4 since they look like they have solid gameplay, but I know Perfect Dark will be riddened with guns that make you look through walls and cloak yourself (Timesplitters:Future Perfect had those types of feautres and it made the game a p.o.s.).
I really have to see more of Ghost Recon 3 to see if it's the game for me. It has great graphics, but the gameplay looks iffy thus far (Cursor runs across the screen etc).
If PREY or GEARS OF WAR were launch titles, I would pick them up - no questions asked, but that isn't the case.
Oblivion looks great, but RPGs piss me off with the open-endedness (You can be really shitty and not even know it after 50 hours of playing and have it kick you in the face later on) and building your character up so you can kill stronger enemies can get tiresome for me. It looks like a great game, though.
This is why I'm waiting until I actually choose a game I want.