I agree with a lot of what Rei40c says, but I own and shoot an LCP, LC9, SR9c, and many others. There are 2 really important attributes of a ccw that is missing here. First is the thickness of the gun. The LC9 is much thinner than the SR9c. That speaks for itself. The single most important thing for any personal defense weapon is the ability to hit your target. I am much more accurate with a gun that fits my hand and I can get a good trigger squeeze on, and lord knows, the LC9 needs it. I can't do that without a mag extension on the LC9. If I can get the same or better grip with a 9 round mag, bonus. It fits my hand and give me 2 more rounds. I still maintain the thinness of the weapon without adding much more length than the mag with grip extension. The first thing I did with the LCP was a +1 mag extension that allowed 3 fingers on the grip. I really don't feel it made any difference in the ability to conceal it and it made all the difference in the world downrange (with a laser, I'm still trying to find the sights).
There are also a lot of people out there that can't afford a multitude of guns and it is rare that you can trade or upgrade without spending money. For those that have the LC9 only or aren't gun nuts like myself, and probably you, the 9 rounder could be an excellent upgrade, provide more versatility to the weapon, and provide an additional 2 if needed.
You can also have the stock mag in the gun if concealed is that big of an issue and pack the 9's as backup. I never pack without additional mag(s) and IMO there is no such thing as too much ammo. In a lot of self defense situations, he who has the most ammo wins (which should stress the importance of hitting what you shoot at).
Given a choice, I'll pack my SR9c before the LC9 because of the reasons Rei40c states, but the LC9 is still small enough for a pocket or my wife's purse and much easier to conceal than the SR9c due to thickness. If I can do that with an additional 2 rounds, I'll do it. Sign me up for at least 2. The sad part is Ruger has dangled the carrot, and not delivered. Very disappointing. I am really getting tired of the gun industry releasing all these great new guns / accessories and not being able to get them for at least a couple of years.