For me, I wish they felt as close in size as they look. Holding the LC9 it feels bigger to me than it looks...and holding the LCP it feel smaller than it looks. Which creates the other dilema for me...the LC9 shoots bigger than it looks for me (meaning I can shoot it almost as well as the larger compacts like the G27 and 1911 Officer sized pistols) and unfortunately the LCP just doesn't have enough grip frame for my long fingers so it seems to disappear in my hand and is really hard to get a consistent grip on.
This also seems to hold true for carry, I find that the LC9 is a lot easier to carry than a Glock subcompact or an Officers model but I find it still pokes or pinches me once in a while...but the LCP just disappears on me without giving me any reminders I am even carrying.
I still carry the LC9 a lot more than the LCP...I just feel better with a gun I can shoot quickly on target, and I can't do that with the LCP at much of a distance at all (talking beyond 10 to 15 feet when you consider having to draw and adding stress).