I don't see any difference between the two on the pitch.
there's some. bonucci has always been weak on 1-on-1 situations, especially when he has to run with the opposition. that's something you can't blame on milan. he got exposed plenty of times at juve and at the nt too. top benatia is a stronger, faster, more physical player, without the very accurate long passes.
bonucci's passing is elite. he might be a proper holding/defensive midfielder. i wouldn't mind him in the squad in sturaro's place.
and about benatia considering offers: he's a mercenary. i don't mind that. if i look at our current squad, i can't find too many players who might have some emotional tie to the club. marchisio, chiellini, barza, pinsoglio, cuadrado are obvious picks, mandzukic also expresses his love towards the club frequently, and that's all. and i'm fine with that. but bonucci, that fucking snake who posed with the curva, could have become a legend here, then shamelessly accepted the milan armband, is the kind of guy i despise from the bottom of my heart.