Toughie. PS3 I'd say is better for platforming/action adventure exclusives but 360 I think is still better overall for RPG in terms of exclusives.
What about online? Is this important or something casual for you? If important 360 has slightly better online but it only really comes into it's own if you regularly play with same friends, etc. Otherwise PS3 offers pretty much the same experience if you simply play casually with strangers and there is no fee.
You know what, why not take a long, careful look at the true exclusives for each at the moment (including 360/PC exclusives as it sounds like your PC is not that powerful) - i.e. Gears, Left fo Dead, etc. on 360 or Uncharted, Ratchet & Clank, etc. on PS3 and decide which has more titles you are interested in.
Given most of the titles on each are the same 90% of the library is pretty much identical so you'd probably be best served deciding which exclusives appeal more.
Or think lateral and get a 3DS or Vita instead. Or wait a bit longer and get both after the price drops all the analysts seems to reckon are coming this year.
Try to be reasonable... its easier than you think...
