M and N have the exact same engine. The exact same horsepower.
N has a heavier dry weight, just from the wing and hardware...
N has squared wingtips, but also plugs in the wingroots to extend wingspan slightly. This means overall it has a bigger wing area. Often this is not much of a benefit because of the extra weight, but I am sure there are some circumstances when you're stalling out with full flaps that this plane will do better than the M will.
N guns are slightly wider apart. The extra plugs of wing are added at the wing root. Shouldn't matter too much, but to be noted as a difference. Hit at convergence and you'll still hit.
Only real differences when folks claim the M is faster, accelerates better, or hangs on prop better, are often due to fuel weight in the loadout. M never flew with AH's ficticious overload-ammo setting, and I don't think they ever flew with 8 guns. Try it with 6 guns+267rpg and the weight savings will really make it shine. M vs N, the M will always have a slight edge, but this is based almost entirely on weight savings. If you get a heavy M and a light N, the tables can turn.
That said, this is just a hypothetical matchup between and M and an N. They're really close enough that (all other things being equal) it will come down to pilot ability. Very generally speaking, take the M to dogfight, and take the N for long range escorts, ground pounding, or multi-role missions.