The PlayStation 3 seems like the place to go for Street Fighter X Tekken, if for no other reason than the extra characters. But the Xbox 360 version also lacks the ability to play the four-player co-op online with a local partner, and for that, one of the producers has apologized.
"Unfortunately, due to time and resource issues, we weren't able to get that feature into the Xbox 360 version and we apologize to those players who were looking forward to it," assistant producer Tomoaki Ayano said on the Capcom Unity blog. "Throughout the development process, there are times when new features are added (and we added a lot of them!) and there are also times where we have to make tough decisions regarding things to cut. This was one of those times."
He goes on to explain that the mode is supported in the PlayStation 3 version, because the team started the process earlier and "found the architecture of their system to be more conducive to implementing it." He closes by promising information on upcoming updates soon, but the tone sounds more like new features than a patch for the 360's co-op mode.
If you have the choice between consoles, the PlayStation 3 clearly received the lion's share of Capcom's attention and care, so it would probably be best to opt for that version. The game is currently available on PlayStation 3 and Xbox 360, with the PC version due in May and the Vita version (with a roster update) coming in the fall.