Noticed the slide lock on the 9 holds it open after the last round. Not a major deal, but nice. The better sights are good too, but don't matter so much in the role the LCP fills for me
At over one pound in weight, the LC9 would fail, for me, the criteria I find most valuable in the LCP...low weight. It's size is also somewhat larger than the LCP.
I have found that I ALWAYS have the LCP with me whenever I am wearing something with a pocket. Easy to carry in a pajama pants pocket and most shirt pockets.
When I am out and about I carry a Glock 23 or Glock 21 IWB along with the LCP in a jacket or pants pocket. When I am home, it is the LCP that is always in my pocket.
I am always able to dress to be able to carry at least the Glock 23 so the LCP doesn't need to meet the demands of a primary carry weapon. (I suspect that should a SD situation arise, the LCP would be the most likely weapon with which I would respond. My hand is usually on it and unless the encounter allowed the time to move to the G23, the LCP would be the one)
So, for me, the LC9 doesn't do the job of either the Glock 23 (larger caliber and greater capacity) or the LCP.