I have both and never carry the 9c since I got the shield. The shield's thinness allows carry options that are amazing (just a shirt!) while the 9c requires pretty much the same as my 9FS or 45c. When given the option, I'd much rather carry either of those if I'm going to be making concessions to a carry garment etc.

Summation: If I carry and a cover garment is appropriate or easy, I'll opt for the FS over the 9c. If I carry and don't want the hassle of a cover garment, the shield conceals just under a normal shirt or in my pocket and no wardrobe concessions are really necessary.

Either way, the 9c is quickly losing my favor and would be my truck gun if I had a non-work truck. I can't put it in my normal one because I work at a place where that's a very big no no and have a protective force armed with M-4s and other stuff to dissuade you.