I'll bet Montreal is better. 18 is legal, and the taxes aren't nearly as bad there (at least not when I went).
But yeah, Buffalo is really not a bad city either. I mean you're just going to be there for medical school which means you'll only be partying maybe once or twice a month, but when you do there are a ton of bars and restaurants to do it in (on Allen, Chippewa, Elmwood, Hertel, Main, etc). I'm sure the nightlife isn't even close to Montreal, but I don't think it's too terrible. I grew up in a town of 10,000 where there was literally nothing to do but go to the local gas station, so that's my basis of comparison.