I think that choice is a fairly tough one to make. Both are good schools, and I think either way you go is a good path to take.
Personally, I would stay in the U.S. I have chosen to go the osteopathic route over allopathy.
As far as the FMG issue; yes......going to Sackler WILL have you looked upon as an FMG. BUT, Sackler does have a good reputation as an excellent medical school. Even outside of NY. One of my closest friends is an OB/Gyn who went to one of the top residency programs here in Los Angeles. Although he was shocked, ( and a little disappointed ), he said that a student from Sackler matched this year for an OB/Gyn residency at his program. This was the first time this program took an FMG, but maybe it might help to start a trend.
Good luck with your choice!