Yeah, I'm also inclined to feel that the height at the rear (butt plate to rear sight) of each of the guns is the most noticeable difference. Whether that quarter inch is worth the additional 2 rounds of mag capacity in the M&P 9c is debatable (or 1 extra round for the M&P 40c).

It might make it more appealing for some folks who won't have to slip their little finger underneath the mag when gripping the gun, or, it might make pocket holster carry & presentation more difficult depending on the size of the pocket opening.

This is one of those instances in which the 'compact' S&W is virtually the same size as the 'subcompact' Glock.

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Ret LE Firearms inst & armorer