Hands down the PS3 for numerous reasons.

You get a Blu-Ray player which on it's own is the cost equivelant of a PS3.

The library though small, will get better as companies sink their teeth into what the possibilities are.

The Cell Processor ( I read a blurb that the US Air Force bought a number of PS3s just for the processor. If the military can utilize this technology, think about potential for games. )

The present and future capabilities of connectivity with the PSP. Heck, you can spy on your house while you're away if you have the camera.

Everything is in one package as opposed to adding externel devices.

Give it 3 years and I wager that the X-Box will be whining like a whupped pup.