Considering you have a crappy PC, if your main concern is rpgs, I would definitely go with the xbox 360 .
You said you enjoyed Fable so theres the big exclusive to hook you. You also get playable bethesda games, Witcher 2, full mass effect trilogy, lost odyssey, tales of vesperia, blue dragon, infinite indiscovery, and if your computer is so crappy it can't even run minecraft, you get minecraft, along with a bunch of nice xbla games.
The only ps3 exclusive rpg imo that you will really be missing out on is valkaria chronicles, but that is more of a stratagy game. Regardless it is really really good, so you are missing out on a great game. I will admit i haven't bought any psn games yet, so you may be missing out on some good ones.
If your main concern is action games, PS3 obviously wins it, it has a far bigger lineup of exclusive action games and the only great games you will be missing out on is a playable version of bayonetta and alan wake.
Platformers, ps3 has littlebigplanet, which i personally hated, but you might enjoy greatly. The 360 has banjo kazooie nuts and bolts which was decent but nothing special. Both systems have a bunch of great arcade/psn platformers though.
