Unless you have no problem paying the Columbia tuition, GO TO SUNY in this case. I can't even see why this would be a decision to make, as a NY state resident it would be silly to pass up an acceptance to a SUNY dental school for a private one. I am at a SUNY school, I don't regret it seeing as I will be less than $100,000 in debt when I am done. You will have the same opportunities coming out of either school (you will be a dentist!), just less money to pay back after graduating from Stonybrook.
doggie - I disagree that Stonybrook would be easier than Columbia just because the classes are different lengths of time. Either way, both schools would have to teach enough info so their students would pass the boards. Doesn't matter how long they take to do it, as long as you learn it.