Prefacing this with the fact that this is the one series that matters to me as a Leafs fan.
I'm going Boston in 6. The Leafs have more talent, more depth, and better goaltending... but they have been atrocious down the stretch and the talent and the goaltending haven't been able to bail them out of their issues. You won't be able to outscore other teams in the playoffs in 6-5 games, either.
Boston isn't the juggernaut the Toronto media is making them out to be, but I think it's more a matter of which of the two completely different styles works better in the playoffs. That advantage goes to Boston for sure.