I haven't carried one but I have held and measured both in person. The size difference between the compact and full size is negligable to the point where carrying the full size would be about the same if the compact is using the finger extension. It's kind of like the difference between a G26 with extension and the G19. Nearly no difference. I'd say stick with the full size or LS.
I have not seen the FN in person yet so not able to give you any feed back on that, I have carried every size Glock frame I can can tell you that in my opinion it does make a significant difference full (Glock 17) vs sub compact (Glock 26). I now carry a G26 with inside the waist band holster and it's very comfortable.
I just bought the FNS9c about a month ago. I carry it in one of the new Alien gear Tuck 2.0 holsters. I absolutely love the gun. Very easy to conceal. It comes with 3 different mags, so you can tailor it to your wardrobe. I use the pinky extension mag usually, but if I was just wearing a t shirt, I may use the compact mag, and it would conceal very very well. The gun shoots very accurately for it's size. The gun was very stiff when I got it, but I just shot it about 250 times, and it's much better now.