I'd go the MMA route for the following reasons:
1. Less head trauma, since there are a lot of ways to lose that don't involve being struck in the head.
2. Less corruption
3. Shorter path to the top
4. Less established... Meaning that boxing has been around for so long that it's very unlikely that you're going to invent a new technique or strategy. But MMA is still relatively new, so it allows for innovation. It's still possible for someone to come out with a great new strategy that's never been seen before.
5. Yes, the money is not as good, but once I make 10 million or so I think I'd have everything I ever wanted. Anything over that is just excessive.
6. If a boxer fought an MMA dude on the street, I like the MMA dude's chances better.
7. I've always loved martial arts, karate, muay thai, BJJ... I'd just rather live in the martial arts world than in the boxing world.