never tried an m40 mag in my S...
then we gotta go shooting. S40 and M357 are the only ones I haven't shot.
now back to the subject. I prefer the M9 mag in My S9. I have FAT fingers and the S leaves a pinky dangling untill I put in a M mag. I only have one S9 mag and just ordered my second M9 mag (thought about modding one of my M40 ten rounders that don't see much use anymore to fit 9mm, but don't trust myself to do it.)
IMHO there's no reason to have more than one S mag, as the only benefit is to help conceal and your only going to conceal a pistol w/ one mag actually in it. For range practice and spare carry mags give me the extra rounds and pinky rest.
Even if you don't need the pinky rest why not have the extra rounds, especially if your ever in a situation where ten wasn't enough to start with.