I really enjoy watching Hummels, but he's a great footballer rather than a great defender.
He does things that looks great, but I think people would find that if they watched him with a more critical eye (like they would with one of their own players) they'd see Hummels makes an awful lot of mistakes.
Now, part of that is because BVB are not an easy side to play CB for, as the midfield doesn't give great protection and the fullbacks are very attacking, but he can be terribly casual at times, is inclined to lose concentration, isn't a good man-marker and isn't particularly mobile.
Now, he could well become a great defender as he matures and if he moves to a team, such as Juve, where it is easier to be a CB, but he wouldn't be worth what he would cost - not to Juve at this point, anyway.
As for Benatia, I like him a lot and agree with Ahmed that he would be a very good option as a Barzagli replacement.