Personally, I would cheer for Montreal - I've never hated them, and liked a lot of their players (from the 60s and 70s more than now), whereas I've never liked Boston or any of their players except Johnny Bucyk.
I know it's not a popular viewpoint here, but I would rather have a Canadian team (except for Ottawa) win the Cup than any US based team.
I am not Canadian so I'm biased, but I hate all the other Canadian teams except for Calgary (of whom I'm ambivalent.) Every single one of those fanbases like to treat Leafs fans like dog crap, not a chance I'd ever wish any of them anything other than irritable bowel syndrome.
I live in Boston. I have never once been treated badly by the locals, but you all need to understand that when a Bostonian gets drunk they literally devolve into a Neanderthal subhuman with a trashy accent and that's not who they really are.
In the end I hate both of these teams equally and when Boston won the Cup in 2011 less than a calendar year after I moved up here from Miami, FL I wanted to lay down on the f***ing train tracks.