Other than your Mom's advice, what make would make you want the 9mm over the .40?
From a features standpoint, the 26 offers one round extra in the magazine than the 27. But is it worth much? From what I was told during my CHL class, most civilian self-defense shootings are over in just a few shots, so the one extra round in the 26 may not be worth much. But then again, there are plenty of people who will take as many rounds as they can.
As for the "stopping power" of the 9mm vs. .40, you should really take some time to read over the material and research that's been done. You'll likely find that "stopping power" is basically a myth. And there is a lot more to the question of effectiveness of a given caliber than the caliber itself: bullet design, loading (i.e. +P, +P+), etc, come into play.
Personally, I carry a 9mm Glock and am comfortable with both the pistol and the round. And my wife may possibly be looking at getting a Glock 26 after she gets her CHL later this year as she wants a pistol and round that she can comfortably handle.
Good luck to you in your search.