This subject has always been a sore one for both sides. Really, there's no good answer on which is "better" because there is some subjectivity in that statement. My suggestion is to do your own research, digging up the actual facts and work from there.
SGU is considered one of the best Caribbean-based medical schools and while I haven't seen the facts, I wouldn't surprise me if they have better success rates and lower attrition than other, less well-established Caribbean-based schools. Still, I bet their attrition rates are higher than any US-based medical school and I am thinking their class sizes are larger as well. I would love to see data from a more objective source (rather than SGU itself) on attrition rates, 1st time board passing rates, average board scores on first attempt, and match list of students on their first go around. These things would be more telling. Nevertheless, if I were to go to a Caribbean school, I'd look toward SGU, though, given it's relative status among the other Caribbean schools.
However, we aren't just talking about SGU. We are talking about Caribbean medical schools in general, many of which are definitely inferior to SGU. Let the buyer beware. Going off-shore for school is definitely going to have it's disadvantages, which should be rather obvious. It seems likely that some will deal with this aspect better than others. Whatever it is now, it seems to me that Caribbean-based schools were designed as a cottage industry, catering to those who could not, for whatever reason, gain admission to a US-based school. I can't verify this information, because I've obviously never attended a Caribbean-based school, but I've heard that many of these schools expect higher attrition, because they typically take a lot more students, taking a gamble on many of them, on the onset, and allow the process to weed out the ones who can't make it. The academic environment, from accounts I've heard, can be a bit tough, because of that. If you make it past the critical points, though, I bet success (whatever that means for each person) is a reasonable expectation.
USDO schools are not perfect, either. They have, in general, their own issues, too. Stick around here and you'll see what I mean. However, if you give me a choice between USDO and Caribbean, it's an easy one for me and I think most people, depending on the relative importance of earning an MD, rather than a DO, and the acceptability of spending time learning OMT.
I guess I'll conclude by saying that I will judge the clinician based on their ability and not where they chose to attend school. If a given school works out for you, then more power to you. Regardless, make sure you do the research and make an informed decision.