Well, MUA Nevis was established in 1998 where as MUA Belize was formed in 2001. I think your best option would be Nevis because the school is more settled and the living conditions are probably better.
The medical school is not exactly in St. Kitts, its on the island of Nevis just south from it. I don't know much about Belize but I've been trying to call the school and i can't seem to get a hold them.
But, do apply to MUA Belize because you don't want to limit your options and miss out an opportunity to get into the MD program.

Good luck

-harleen