this is seriously a tough decision, in my opinion.
you may say you want to do oral surgery now, but to be honest, you have no idea what to expect out of dental school. i can't tell you how many people i've met, who have the grades for oral surgery, but have gotten to the point where they have grown to like general dentistry even more than OS. i've also met people who wanted to do OS, but realized they couldn't do as well as they wanted to get the grades for OS. but, in any case, some things you should consider.
1. you got into columbia (congrats--huge accomplishment). clearly, that's a testament to the kind of student you are. with that said i'm sure you could do well at BU and still get oral surgery.
2. the curriculum difference--any medical school integrated curriculum prepares you better for the cbse/oral surgery exam. not to say that it can't be done otherwise, but it'll make preparing for the exam a heck of a lot easier.
3. the grading systems--columbia is H/P/F, where as BU isn't. what do you prefer
4.. pre-clinically/clinically , both schools are pretty average. columbia doesn't have the best reputation from a clinical perspective. and i'm not making that up... that's from talking to resident directors and professors who have taught there. say you discover that oral surgery isn't for you, and you want to pursue general dentistry. then what? you'll be at a disadvantage clinically.
5.. cost-that amount is a huge thing to consider. yes columbia is 300k, and it is in the most expensive city to live in in the U.S. is it worth it to you? BU is throwing a lot of money your way, and that's something that i wouldn't take lightly.
when all is said and done, i say BU. between the $$ you'll be saving, and keeping your options open from a general standpoint, i feel like you can't turn that down.
edit: and i want to say this---columbia does not make getting oral surgery easier. that is a very bad misconception. they churn out specialists because, first of all, you're classmates with some of the best dental students in the country (statistically). so naturally, you'll have your specialization rate at a very high %.
the boards are P/F. the only other things that really come into consideration: 1. your cbse exam score 2. extracurriculars---externships, research, shadowing 3. letters of rec