This one is real easy for a Habs fan.
Nothing a player from the Leafs has ever done comes close to the two month long persecution of Max Pacioretty by the CURRENT CAPTAIN of the Bruins, Zdeno Chara, culminating with the terrible injury at the stanchion. The subsequent denials and further disrespectful comments from Chara, Mark Recchi and the top brass of the team only made things worse.
Nothing Randy Carlyle has ever done comes close to the multiple seasons long torrent of disrespect that comes from that turd Claude Julien, including his constant whining about embellishment and creation of excuses for every loss, as well as his sending goons out to beat up non-fighters.
Nothing any Toronto broadcaster has ever said comes close to the anti-Canadiens vile put on the airwaves countless times by Jack Edwards, whom the Bruins organization has done nothing to muzzle.
This stuff is so lopsided that just on the non-hockey scorecard alone, Boston MUST be the team that is "hated" more.
As for the hockey side, you almost always want the underdog to win in any out-of-town series, as it makes your own team's trip to the Cup easier.
So, on both dislike-level and hockey-level, I see no alternative but pulling for Boston to lose. The rest, that it should go long, that the Leafs might come out of the series hurting, these are all minor details.
Cannot bring myself to ever cheer for the Bruins against any other team, unless it would be in some future year where they are considered an easy team to beat, and they are in a position to upset a Cup favourite that we are afraid we have little chance against.