20mm are good, but the ammo load is limited and once they're gone you just have the 4x.303s (or 2x7.7mms in the case of the Zero) to go with, which won't do much more than tickle.
I'd take the six Browning .50cal of the F4F-4 over the mixed armament of the Spits and Zeros any day of the week. The Ma Deuce has about the best ballistics properties of any gun in the game (said this once on the board and someone remarked "WHAT ballistics?! They're lasers!" lol!) and is about as to point-and click of any gun there is. Get your target in convergence range and they hit HARD. I set the Brownings in everything I fly to 200yds, and I've sawed wings off P-47s with a half-second burst at that range.