I was stuck between the two for the #1 slot on my rank list. Initially I focused purely on the cardiology match between the two programs.
Somehow I "saw the light" and realized I needed to be at a program where I felt I would get a stellar residency training and walk out more seasoned as a clinician than at other programs.
Once I matched into a program and started/settled in, I would THEN worry about fellowship (at both Sinai and NYU, you will match into a fellowship in the field of your choice. They both do very well in this regard).
My advice: figure out what type of residency program you want to be at (cush? hard work? diverse patient population?) and worry about the subtleties of fellowship match (between similarly-ranked programs) when the time comes.
(IMO Mt. Sinai is a fantastic program with a great group of residents and a very caring, hands-on PD... however, after comparing both housestaff programs with second-looks etc. I decided to go with NYU as it was more of what I was looking for).