For your room I would go with M22s if you have a good sub. The M40's big strength is that they seem to be great all-round speakers and can handle full-range music without a sub. M22s don't have the same bass extension but will be smoother and more clear in the midrange. Both differences are fairly small but if you have a good sub I would go with m22.
If you look at the soundstage reviews you will get a pretty good comparison. Relative to the M3, the M22 is felt to sound a bit better at a higher price, while the M40 has much smoother bass but is a bit less clear in the midrange than the M40.
If you were looking for music speakers and weren't into subs, the M40s are really hard to beat (I would have bought some but came into a bit of extra $$ and got M60s instead) but if you have a good sub I would lean towards M22s.
http://www.soundstage.com/revequip/axiom_millenniam22tise.htm
http://www.soundstage.com/revequip/axiom_millenniam40ti.htm 
