There is no simple answer to that question. What exactly is "better"? Lower attrition rate? Higher pass rate on the boards? Better residency placements?
The first thing you need to do is decide between the allopathic and osteopathic approach. If you believe the osteopthic approach fits you better, than SGU isn't an option. If you prefer the allopathic approach or for some reason cannot deal with the implications of being a DO, then St. George's is a very good option (assuming you cannot get into a US allopathic school).
Personally, I prefer the osteopathic approach and so that is what I am going for. I will say this, if I wanted to be an MD and couldn't get into a US program, SGU would be the first place I would try to get into. IMHO, it is by far the best Caribbean medical school out there.
Good luck.