Kitty, no question about it. Frankly, the MJ-Peter relationship is about 40 years old now. It's just a tad bit played out and boring by now.
The Peter-Kitty relationship is fresh (which, to me, is exactly what the Ultimate universe needs to be about, fresh ideas that can't really work with 40 years of back history, not just recycling stories and relationships from the 616 universe), it makes perfect sense, and it just provides for more interesting stories. I mean, how many times can "villain X has escaped, and is kidnapping MJ, and only Spiderman can stop him/her, oh noes!11!11" be told? Because that's really the only MJ story out there, at least it's the only one I've ever seen.
And, to be honest, the relationship just makes sense. MJ needs to be protected. Kitty can protect herself just fine. MJ has never really been much of a social outcast, her character was introduced as a member of the "in-crowd". Kitty is the definition of "outcast", and ergo can probably relate more to Peter. Kitty is just able to relate to Peter in ways that MJ can't, and she's able to date Peter Parker without Spider-man messing things up.
And I really don't think the "He loves MJ, he's only with Kitty because he needs stress relief and doesn't wanna worry" argument has any real merit. There's nothing in the books that I've read that supports this, at all. The only way to really see it like this is to just want the "traditional" Peter/MJ thing back so badly that you begin twisting what you read to fit that. Does Peter love Kitty right now, or vice versa? No, probably not, but they're extremely fond of each other, and just like a real relationship, the love will develop from that, it isn't (and shouldn't be) an instant thing. Does Peter love MJ because he talks to her? No, she's still his friend, and one of the few people he can confide in about BOTH aspects of his life, and she doesn't have to hitch a ride on a jet to see him. It's as simple as that. He's getting over MJ, and he's falling in love with Kitty.
So yeah, keep the Kitty thing around. To be honest, I wouldn't mind too much if they just killed MJ off, or at least kept them apart for a very, very, very long time.