of the two: pc.
if i had to pick between pc and psp(or other popular portable system): other portable system.
Reason: the xbox360 is supposed to be like $400 for the good version. A new pc that blows this one out of the water is roughly $750. Upgrades to this computer to blow THAT new pc out of the water would also cost around $750. Pc's can be upgraded fairly easily, but I can't exactly expand the harddrive in an xbox without torturing some end user license agreement or warranty.
Why the portable system over pc though? Simple. I can't put my desktop in my car, or play it at school. I could if it fit in my pocket though (and no, I don't mean play the game while I'm driving, I mean while my friends or parents are driving. Just had to clarify.).