The Glock subcompact is smaller but the M&P compact is designed for more comfort.
Here is a link to a comparison photograph of the two pistols:
http://glocktalk.com/forums/attachme...0&d=1216400653
If you are really looking for an easier to carry 9mm semi-auto pistol you may want to look at a Walther PPS. The new PPS is surprisingly thin and easy to carry