Hi Pierre,
BU math is quite a small department (at least in the pure math part) in terms of number of faculty or students, the advantage though, is you can take classes and seminars in neighboring Harvard and MIT. But generally it's almost impossible to take advisors from MIT or Harvard.
The life in Boston, as Multiple Infusions pointed out, is pretty nice. But generally students don't own cars. the housing rent is just usual as in any major American cities. the public transportation is convenient. The winter is very long though, and can be really cold.
It seems that you are admitted to UNC now? I think UNC ranks top 3-top5 in stat departments in USA, but of course, the most important thing in grad studies is the advisor, not just how big the department is. And I think you can take this chance to visit Boston and then make your decision!
Best regards!