Bookmakers are currently pricing this game as follows:
2.40 - 3.25 - 3.10
It turns out that despite Napoli being the favorite in the title race, they are the underdogs in this particular game. I don't even know if it's fair to rate the current champion and race leader as 3.10? Plus, Milan is far from being the strongest competitor and is very far from its best days. For some reason, I'm starting to think that betting on Napoli is the right choice, or that I could take the more reliable Napoli Double Chance.
I don't think it is fair rating on Napoli. Currently they are a better team than Milan clearly. Especially in terms of offense. Besides when we check the head-to-heads, we can see Napoli beat them home and away both last season.
Therefore I don't see any logical reason to underrate them in odds.

Milan might seem to be improving recently but I wouldn't trust them in this game. Currently it is 3.20 for Napoli and seems worth a try for betting...