I posted this a couple of times but never fully elaborated. In a way, both these teams are subtle anti-thesis' of each other. While possessing some similar attributes, they thrive on different things while being able to tap in to a lesser version of what the other team is better at.
Boston plays a heavier game than the Leafs. Their entire top line is filled with guys who can cycle with anyone in the league and the rest of their line up can do that too, just not as good as Marchand - Bergeron - Pastrnak. They have good speed all around and are one of the faster teams in the league, blending a heavyish game with good team mobility. They aren't the most mobile on the back end sans Krug and maybe McAvoy, which is probably the main exploit that the Leafs should try to take advantage of.
Toronto is a wave after wave speed team. Top to bottom you have Matthews, Nylander, Marner, Kapanen, Johnsson etc. Who are all fast skaters and absolutely burn it with the slightest bit of open ice. The first line in particular is quite good at cycling, just not as good as the Boston first line at that, but they're faster overall, channelling the main difference between the way the two teams play. On the flip side, Toronto's D is far more mobile, but is easier to suffocate with heavy play that Boston thrives on. This is what the Bruins should try to take advantage of.
Essentially, the series will be decided based upon if Boston can neuter the Leafs speed game more than the Leafs can neuter the Bruins ability to start a heavy cycle, the two main weaknesses of the two teams. I would surmise to say the goalies are quite even, posting very similar numbers. In a match up to match up case, Andersen wins out slightly since he wins so much against the Bruins, which could be an X Factor. That among many like -> Matthews vs Bergeron; Who goes on a scoring outburst first? I like the Leafs depth of elite talent spread out in this case, as the Leafs Kadri-Marner line is my personal X Factor for the series, where even if the top lines neuter each other, Marner could feast and find this series wide open. I really like this match up for it's storyline and the fact that Atlantic is from an only top teams perspective; the strongest division in the league.