I think one has to factor hand size in to the equation. I have glove size XL hands and thought the 8-round (9mm) would be the logical choice. I've never liked the dangling pinky on small autos and it has kept me from owning certain guns, but a comment from YouTube sensation Hickock45, who is something like 6' 8" and has giant paws, made me rethink my logic. He commented that he actually preferred the two finger grips on subcompacts.
When I got my Shield I looked at the issue a little closer. Turns out, if your hands are on the large size, the small diameter of the semi-staggered Shield mag well is a little too small. When you add the longer mag that allows the pinky to fit, you actually keep the middle and ring fingers from grasping as far around the grip, as is possible absent the pinky finger. You have to try it for the explanation to make sense, but this is why I like the 7-round mags better.