The main concept behind the M3 over the M2 seems to be the base bump that makes it well suited to use in a 2.0 setting. The M2s really do need a sub to fill out the low end.

If you are using a sub I'd say go with the M2. I have both and the M2 is more forward and precise. Mind you, I'm sure not getting rid of my M3s, which sound great w/o the sub.

M80s/VP160/QS8s/EP350; M22s; M3s.