Disclaimer: I have not heard either of these speakers. I am only re-iterating the information I found that helped me make my decision.
Not too long ago, I was trying to make the same decision as you: same budget crunch, same mostly musical needs and plans for the future. My room is actually a wee bit smaller -- about 11x17. I went with the m22s and they might arrive as soon as tomorrow.
From what I've read on this forum, the m40s are considered to be a more laid-back speaker than the m22s. So while it is of course a matter of personal preference, the majority seem to prefer the sound of the m22s, especially for music. And although I plan to get a sub somewhere down the road, what I've read indicates that the bass response of the m22s should be sufficient for my small room for the time being. I live in a poorly insulated student ghetto apartment, so loud bass would not be appreciated by my neighbors.
Are you really planning to never get a sub? Most people seem to think that subs help a lot -- even with the really big tower speakers -- and the frequency response of the m40s is only 40hz-22khz +/- 3 dB, so you'll be missing out on some bass, especially in movies.
