If you are going to be living in base, it honestly doesn't matter. I've been based at both, they all have pros and cons. JFK is more junior, and you will move up a little quicker. JFK is also the largest Airbus base, so more open time and variety. BOS is a smaller base, but I really enjoyed being based there. I'm not the biggest fan of NYC and I hated having to cover LGA or get send to EWR/HPN/etc while on reserve. If you are going to be living in base, I suggest looking at BOS. Lot's of guys live in southern NH that drive down to BOS.