Thats not really true of Marvel. Sony sponsors the major tournies and as such they're played on PS3. Happens a lot with Sony actually and they tend to buy exclusives in fighters to get a leg up on nonexclusive titles to essentially make them exclusive cheaply. SFxTekken comes to mind with Kuro, Toro, Cole, BBA Mega Man, and Pacman ALL being exclusive and with MK9 getting Kratos exclusively on PS3.
A lot of fighters run better on 360 because there tends to be some frame drop on Marvel and a few other fighters on PS3 likely due to the split processing of the system, but Sony money on titles and tournies sort of buys a lot of the fighter crowd to PS3.
I only own a 360 BTW. Just stating what I've experienced and I gotta say it blows to be a 360 guy who plays stick knowing you'll have to pay to dual mod your gear or buy another stick entirely just for tourney play 9 times out of 10.
Well first off, the last time they tried to give a fighting game on Xbox an exclusive character, they ended up with Spawn in Soul Calibur 2, and the funniest thing is, he was originally going to be in all 3 versions. (you can still find code for him if you use AR on PS2 or GC)
It's pretty ironic considering those same developers made sure the games they developed for the PS2 all ran a-ok from 2002 onwards, and the PS2 was "harder" to program for than any console released in the last 10 years. Look up the architecture for it, it's inconsistent as hell. There's not really such a thing as a easy as hard system to program for, it's generally who's first out of the gate and who will therefore get acquainted with developing for it year in year out. When PS2 dev kits were first shipped out to major publishers, everyone was bitching relentlessly about them, but that's because they were coming from the Dreamcast which had very PC/Windows-esque architecture on it's dev kits, and everyone was able to figure out pretty quickly how to get the most out of it.
One more thing, I know how you feel about not having a dual-mod on your stick, here in the UK, every single fighter is ran on 360, no questions asked, although it's nothing to do with running any better, the reason why they use 360s is.. pretty laughable actually.
The PS3 version was at TGS.
Interesting.. but then that might only be because of how poorly received the 360 is in Japan.