Kessel didn't really play on a line with Crosby other than the PP. Prior to last year he actually didn't play with Malkin that much on 5v5 either. He is bad defensively in general, not that domi is a shutdown defensive guy but he is miles better than Kessel defensively. Overall kessel is the better player, not by a massive gap, but he is better.
I'd still rather a 24 YO Domi over a 31 YO Kessel.