Bass extension and overall output are the salient differences between the M3 and M2ti. The M3 will extend lower (helpful in getting a more seamless blend with a subwoofer) and play louder than the M2ti.
Midrange is excellent with either speaker. Very neutral. However, the M3 has a subtle reticence in the midrange, a kind of "laid back" quality that many may find more enjoyable. I don't find that to be the issue with the M2ti.
I don't think you can go wrong with either speaker. I will simply reiterate my preference for the M3 as the least expensive Axiom monitor that I could live with without the use of a dedicated subwoofer. With a sub in hand (or more likely sitting in some corner - lol ), I would have have no qualms going for a pair of M2tis for a room the size of, say a typical bedroom.
Good luck!
-alan
