No big difference if you fly C-class IMO. NW will give you more miles, AF lower Elite thresholds. Both will give you free domestic upgrades on NW. AZ has a direct flight from BOS to MXP, but C-class is no good on Alitalia. Connecting via AMS, DTW, or CDG gives you decent connections to Italy. Given that you travel C-class a lot, I would go with NW if you can make 75k EQMs per year. If it is 40k ish, I would go with AF.
Award availability in Y is very good for AF Elites, in C-class there shouldn't be any difference between NW and AF; reportedly AF Plats sometimes get additional availability. Keep in mind that AF has a 747 on the BOS-CDG route until May, so O-class (C-class award) availability will be low until they switch back to the 2xA340.