Murray and the Pens PP are the keys to the series for me. Deserved or not, Murray will likely be the guy in Game 1. I can imagine a scenario where he is just so-so in game 1, and they go back to him in game 2 and he is flat out bad. The Habs don't score much, but they get a ton of rubber on net, and Murray is prone to giving up softies, so a high shot volume is trouble.
As for the PP, there are typically less PPs in the playoffs, so the advantage the Pens have on special teams is not quite as pronounced as the stats would suggest. We also know that the Pens can be a pass happy group and turn what normally should be a good PP into an awful one. PP % of < 10% in last years playoffs against the Isles was a huge part of why we got stomped. 1-2 more PP goals would have gone a long way. When an opponent knows your PP is ineffective, they don't mind clutching and grabbing a bit more aggressively as ES.