If we're talking right now, today...i'd go with the Leafs pretty easily. The Penguins are not playing well, and more than anything...DeSmith and Jarry just isn't doing it for me. That's not the sort of goaltending i'd bet on, gun to my head to win a series for them. The Penguins don't have the defence (and haven't for years now) to get away with anything less than really high level goaltending. Even if we're allowing for Murray returning...i'm not so sure on where he was at to begin with, and he'd be coming off a lengthy absence with plenty of rust likely.
I go with the team that's really clicking offensively, has a goaltending playing well, and just got a significant extra piece back in the fold in Nylander to bolster their offense even further.