I think there's a little bit more pressure on Boston. The Leafs are already "dead man walking". Win or lose, there still likely going to be significant changes - Shanny and Keefe/coaches gassed; maybe some roster changes. Honestly, an appearance in the finals is the only thing I can see saving Shanny and Keefe. And maybe still Keith Pelley moves on from Shanny regardless. So, at some level, the Leafs have nothing to lose, cuz (as Dangle said in his LFR channel), this is already Zombie-Leafs.
Boston is not only playing the Leafs, they're playing against their recent history. Plus, they're on home ice.
All that said, I still think Boston gets back to their game a little, and dusts off the Leafs.