The M9 is the first handgun I ever bought, and the S9 is my second. The M feels a little more natural in my hand because I have long fingers; even though the grips are almost the same, my fingers seem to wrap around the S more and trigger finger placement is a little more awkward. Also, I have to be a little careful on magazine changes with the S or I can pinch the side of my palm between the mag and the grip. 8O
The S feels heavier in my hand than the M, but I think it's psychological because with the smaller size S I just expect it to weigh less when I pick it up. I thought the M was quite accurate (for me) from the first time I fired it at 10 - 12 yards, but with the S at first I had trouble hitting a 10" plate at 10 yards. After I worked on my grip some, I could shoot the S pretty much like the M. I qualified for my CCW with my M, and at 7 yards the instructor could cover the pattern from 50 rounds with one hand, which really pleased me since I had only put 110 rounds through the gun up to then.
My S has bonked me on the head with brass once and nearly put a case into my shirt pocket once; the M has been more polite.