Here is a compatability guide for the Xbox 360:
Xbox360
The PS3 is quite a grey area in terms of compatability, most of the time they are compatible but some PSone/PS2/Bluray/DVD's etc, are
sometimes not.
What console you get depends on what games you want.
Xbox 360 has stuff like Halo:
YouTube - Halo 3 Trailer
Call of Duty:
YouTube - Call of Duty 5: World at War Trailer - Weapons Showcase
PS3 has stuff like Metal Gear Solid:
YouTube - METAL GEAR SOLID 4　Theatrical Trailer 2008 extra
Little Big Planet:
YouTube - PS3 LittleBigPlanet Trailer - Official
The Xbox 360 is a lot cheaper, it's currently cheaper than the Wii around here now, for the basic version, so if you are a casual gamer than maybe you wouldn't want to spend that much.
The Arcade version of the Xbox is the basic version, but it doesn't have a hard drive. You can buy one seperately if you want it. The Premium version comes with pretty much everything you need, including the hard drive.