Why are you having a tough time deciding? This should be a very easy decision. They are equal almost in every way...reputation, ability to get residencies, curriculum, grading, 3 hospital system during 3rd year (although for NYU all 3 hospitals are within walking distance to one another), even the order of their course schedules. You will not be at an advantage at either place, unless you know you want to go to one of them for residency.
So, with that confounding reputation stuff out of the way, just go with your gut...
1)where would you rather live: upper east side or downtown?
2)where did you get a better feel: Sinai (kinda touchy-feely, more laid back) or NYU (a little more traditional)
3)which housing did you like better?
4)Are you interested in research? if so, look at the faculty projects and see if there is one that you REALLY want to be a part of and contact that person. Instead of hoping you get an interesting, you have the chance to pick your school and create your own opportunities.
5)which lecture hall and hospital do you find more aesthetically pleasing?
Some of the things I listed may seem trivial, but they are so evenly matched that these are just as important as anything else you can come up with. It may seem like a huge decision, but its really not...the opportunites presented to you and even your life during med school will not be that much different at either place.
Good luck!