Definitively Kitty ... a few reasons for this
MJ and Spiderman breaking up and getting together again is really old; Ultimate Universe, new take on it, let's have something new for a wile.
Peter is having problems right now with the Peter Parker part of his life:
- Gwen died
- Aunt May threatening to cast him out and their relationship getting steadily worse
- School going bad, etc etc
Basically, most things in his "normal" life just isn't working out for obvious reasons, he's been depressed about it, and he has decided to try to do something about it. He did a reversal on the "spiderman no more" and said "peter parker no more" instead. Also, he said, he couldn't take it if he had MJ's life on his conscience as well and that seemed steadily more likely to happen as she ignored his warnings continously.
The big difference is that, obviously, with Kitty he wouldn't feel responsible. And also, Kitty is dating Spiderman, not really Peter Parker. They are going out partly as superheroes, and just a bit as "normal" people. Considering Peters depression and "Peter Parker no more" it just makes a lot more sense that he would be involved with someone that fell in love with Spiderman first. And well, he didn't seek it out, he got hijacked into it and all his reasons for not dating MJ are not really valid for not dating Kitty.
Besides, Kitty and Spiderman is a good pair from a "team" point of view as well; well connected, interesting more than superpowerfull powers, and basically very good slightly naive young people. Could get up to a lot of interesting things together; and in the X-Men, Kitty would just be overshadowed by other more interesting characters. At least in my opinion.