im biased in that im angry/in love with both of them. a couple of months ago i would have said columbia, but i believe mssm is probably a nicer place to be and it is a bit cheaper (if i recall correctly), you can't go wrong with either, but im thinking im slightly tilted in facor of sinai (that is aside for prestige). if you feel that the prstige/name of your med school is important, then definitely columbia. their name certainly carries more wait.