To be honest, as an owner of both systems, all the good RPGs are multiplat anyway. The only one I found to be any good exclusively on PS3 was Valkyria Chronicles.

360, despite many people not thinking so, has some very good exclusive RPGs. Stuff like Lost Odyssey, Last Remnant, N3, The Witcher 2, Fable II. It's a matter of taste.

The big RPG games like Fallout, Skyrim, Borderlands etc, I find to be more glitchy on PS3. It is however, a matter of taste. X360 does have an absolute dire backlog of older games on it, so PS3 definitely wins there. I don't know really, I like both!