I am not at B6 or DL, but in the vast majority of cases I would say DL. However, Boston might be one of the few exceptions. Boston isn't going anywhere for B6 and commuting to sit SC in NYC covering three airports sounds like a nightmare, regardless of how good Delta's reserve rules are. I think your decision largely comes down to what you think of B6 and whether you could be happy there without wondering "what if."

I know this second part won't answer the question you asked, but I feel that it is applicable to your situation. If it is an option or becomes an option, I would consider going to AA. Though AA's BOS base is tiny, it would give you an option to drive to work at a legacy. There have been multiple BOS slots in many of the recent new hire classes and, to my knowledge, they have been going junior within classes.

I honestly have no idea what seniority projections would look like at B6 (probably not favorable with such a young group unless you bank on attrition to legacies), but I do know it will take roughly 20 years to hit 50% at DL. The unfortunate fact is that the ship has sailed on getting in at a "good time" at DL and you are almost guaranteed to be on the back end of their hiring wave. At AA, you could be 50% in 8-10 years based on retirements.

I just think AA would give you the best balance of what you're looking for. You could be in BOS for likely your entire career, or commute to NYC, etc. if you get an itch for more diverse flying. But at least you would have options.

I am aware AA management hasn't been close to DL's in recent years, but AA is trending in the right direction and the operational differences should be minimal over the course of a 30+ year career.