At the risk of echoing what Vlatko just said, Cassano hasn't really proven much, sure he might be the more talented player, but is he the better player? That is extremely debatable.
When you think of it, there stories are a little similar. Cassano at a semi big club ala Roma did very well and was lauded as one of the hugest talents in Europe let alone Italy. Similarly Benzema was great for Lyon too. Both then went to bigger clubs, with higher pressure and bigger expectations pinned on them, and both have their attitudes not their abilities to blame for their failure to deal with a tougher environment. Cassano went back to a smaller team(Samp) and he's back to his best again, with minimal pressure on him and with him being the big fish in a small pond (excuse the overused cliche), i firmly believe and i think you will agree with me here, that if Karim would take a similar career path and go back to Marseille or another Ligue team, he'd probably excel again.
So really, what is the difference between the two other than the obvious fact that Cassano is in better form?