Short version, Neal has more physical edge to his game but Kessel is faster.
Long version, Neal's offense comes from his positioning and his shot, he is the prototypical "sniper". His physical edge is not productive; he takes himself out of position to throw hits and starts **** he can't finish, gets riled up in a way that detracts from his game. As opposed to someone who is productive in their physicality, making it difficult for opponents through hard work and tight checking, winning board battles, that's not his thing. Not that it's Kessel's either, far from it, but he's not a downgrade from Neal in that regard. Neal probably has the better shot but he relies more on his teammates to get it for him and then finishes. Kessel uses his skating and slipperiness to create his own chances. There's a lot of the young Ovechkin to his game, carrying the puck down the wing and just blowing past guys. He needs linemates who can take advantage of the time and space he creates with his speed and the threat of his speed to cash in those chances.
For that reason I think he is a better fit with Crosby than with Malkin. Malkin likes to dominate the puck and bull his way around the ice; he meshed well with Neal because Neal would play off that to find a scoring spot and wait for his shot. Crosby is so hockey-smart that he will use the opportunities Kessel creates off the wing to read the defence, find space around the net and finish the chance when the defence reacts to Kessel.
