I think that FIU will be a tremendous school when it is up and running. However, if you have been through the application process there, you have seen what I mean when I say a school can be impacted by its own growing pains. I had an excellent spot on the waitlist, but ultimately pulled myself off b/c of the following reasons:
1) FIU is still figuring out how to do things. They have a lot of hand-holding from other Florida schools, and their HELP program sounds great (I hope other schools pick it up). But even established schools have growing pains when they merely tweak their programs. They have years to get this right, but you only go to med school once. If they misstep, do you feel ok if it is done with your education?
2) One of the benefits they touted of attending FIU was that since it was new, there would be a lot of leadership opportunities to start clubs and programs. In one regard this is great. But part of me also is daunted by med school and the thought of having to start everything I want to participate in...and only having ~40 classmates to recruit from. I've never been the type A, class-president type, so I doubt I will feel like that once I start working my butt off in med school. There is something to be said for being able to walk into an already-established club/organization and being able to participate in events/etc. without having to be the leader who plans them. I fear I sound lazy, but I think I'm trying to be realistic. A lot of leadership is passed down from year to year...it's going to be really hard starting from scratch. In some regards, I think it might mean that some opportunities will be limited by the time and motivational constraints of the small # of incoming students.
3) The professor to student ratio. Will be AWESOME since the school isn't full of 4 classes. No question that you will be remembered by your instructors and that it is great that if they want research help, you are more likely to get to participate, etc. My experience working for med profs who do research, however, is that they take about a year to get "set up." Fortunately they are recruiting some profs from within other PhD programs and who may get to stay at their current hospitals...but I would double check that your expectations within this regard are realistic, if you are considering this is as a "pro." I would also realistically consider their ability to retain profs--I don't know what "usually" happens at new schools...is there a possibility that they will have a high turnover rate and those profs with which you greatly bond during the 1st two years will be long gone before you graduate? I would look to the newer D.O. schools for any predictions. Going to a new program to get a promotion in rank in order to then move laterally (keep that rank) to a school of higher status is a relatively clever way of getting a promotion in some fields of academia. What are they doing to ensure professor retention? (Lack of retention makes it hard to strengthen your curriculum and thus reputation, too).
4) In which environ (both natural and psychosocial) will you be happier? I don't think this is a negligible factor and the influences taken into account will be different for everybody. Do you do better in sunny environments and have mild seasonal affective disorder like me? Maybe you'll actually do better in school if you go to Miami instead of Ohio. That could be a worthwhile trade off...I'm a lump in the cold and have definitely gotten worse grades in cold seasons. I was questioing LA vs. Miami, but I loathe humidity and annoying members of my family who live in FL...it kind of cinched the decision.
In spite of what I wrote, I don't think you're shooting yourself in the foot if you go to FIU. I think it will be unique, and I don't think there are any of the concrete negatives surrounding any of the new M.D. programs that there are surrounding some of the new for-profit D.O. schools (at least they have the foresight to start with a small class!) As said, starting up in FL, at the same time as another school, they have carefully and critically planned this school. I think that certainly, if you want to have a career in underserved/minority/immigrant medicine, I wouldn't toss this acceptance too quickly. I believe that FIU will become a strong force in that field...the question is whether or not you are confident it will offer that force by the time you are graduating.
Best of Luck!