Some sites overstate Boston's growing season because they base frost dates on airport readings at Logan Airport.
Logan benefits both from UHI and being located out in the bay. It is not really reflective of the growing season in MA. For example almanac.com gives Boston a longer frost-free season than Nashville. Even if that were technically true it would not be a good reflection of the climate nor of the vegetation and timeline for fall colours in New England vs Tennesee.
Boston probably does still have a longer growing season than Bern. I don't know anything about that climate just wanted to point this out.
I would pick Boston's climate because it's a bit warmer for Apr-Dec