It's SO hard to compare, they are completely different players.
Crosby is more of a grinder, in that he's amazing in the corners, and wins battles along the boards all the time. I feel that is his biggest strength, that he can outmuscle and outwork anyone down low with that low centre of gravity.
Kessel is a much more flashy player. He's the guy you'll see undressing defencemen on a nightly basis. A real gamebreaker in every sense, he has the speed, shot, everything. It was real impressive to watch him today, with the defencemen backing off so much to compensate for his speed.
Right now, I would not be able to say who is better because they don't play alike at all. Crosby is better in traffic, Kessel is better in open ice (from what I've seen)
Personally, I see Crosby's game translating better to the NHL because the league is based on cycling and corner play right now, where he is excellent. I'd love to see both succeed, cause they're both extremely exciting and would definitely play a big part in reviving the game.