As Jay stated between the M2 and the M22 there's a bit of loss on the low end response and a bit of efficiency loss on sensitivity rating as well.
The response difference is fairly subtle (only 10 Hz) and is mostly irrelevant if you're crossing your receiver at 80Hz.
The efficiency difference should be well compensated by your receiver setup and would only come in to play if you're looking for extreme volume levels and you're tapping out what your receiver can deliver amplification wise.
On a side note my in/on wall M2 was just delivered 30 minutes ago - perfect timing for my birthday today!
I'll be comparing my in/on wall 150 to the M2 and will post impressions in the coming week. Of course it will be subjective impressions as I can't do a true double blind, but I'll keep the opinions as honest and unbiased/uncoloured as best I can with level matching, same source material etc...