Can you get an MMA trial? Seems like about 80-90% of people that start training in MMA (at least our gym) quit very quickly. It's not for everyone. It would suck to waste your money if it's not for you.
I think that MMA would get you in better shape, but I've never done cross fit so I can't really say. MMA is a lot more exciting than just working out though, for sure.