Kessel is a superior goal scorer and is one hell of a playmaker. Unless Hall is able to become a great playmaker, Kessel has the edge now and in the future.
Hall is an underrated playmaker, it's a dimension he's added to his game since he's come into the league, and the results really started showing this year (8th in the league in assists). A lot of his "playmaking" is being able to back off defenders and find or create chances for his teammates. He's also a very good passer; I've noticed especially an uncanny ability to find teammates with hard saucer passes.
Leaf fan going with Kessel. It's very close, Hall is a lot younger and likely has more upside. At this point though, Kessel is a player who can adapt to any line and still be a PPG player. He can score, but can also take a back seat and be an elite playmaker if needed.
I'm going to go with Kessel because his team is starting to have success. I don't think Hall can replicate what Kessel does for this team. It might change because Hall is younger and likely has a lot of untapped upside. However, Kessel is also only 25.
Comparing Kessel to Hall at the same age there is no argument... comparing them now I am still leaning to Hall because he has more puck hunger/will to win. Both have great shots/speed... but Hall has more grit to his game.
He gets knocked for it due to the risk of injury of his playing style, but it's what makes Hall special.