Pretty much any fighting game for PS3 is available on 360, so you don't need a PS3 for fighters. Wii will be getting Tatsunoko vs. Capcom soon.
PS3 has very little exclusives in the way of RPGs. The only one I can think of off the top of my head is Valkyria Chronicles, so you're kind of covered for RPGs (unless you really want Final Fantasy Versus XIII when that comes out next year). Still, the upcoming PS3 version of Tales of Vesperia is getting a load of extra content (characters, etc.).
As for platformers, PS3 has Little Big Planet, which is absolutely amazing, but then again Wii has Super Mario Galaxy and is getting Super Mario Galaxy 2 and New Super Mario Bros. Wii.
I own all three systems and I love my PS3, but I'd recommend getting a Wii first.
When it really boils down to it, look at the exclusives: do you want the PS3 exclusives like MGS4, Resistance, Killzone, LBP, Infamous, Uncharted, etc., or do you want the Wii exclusives like Mario Galaxy, Super Smash Bros. Brawl, Tatsunoko vs. Capcom, etc.?