The Comp is really big. Personally, I think it looks ridiculous but I know a lot of competition shooters who use them and love them. The C is more practical for carry and, in my opinion, looks better.
If you aren't doing USPSA, Steel Challenge, etc., I recommend the C.
Edit: I run the 507K (which is even smaller than the 507C) on my carry (PCR) and competition (SP-01 Tactical) guns. It's a bit small for competition but I prefer to have the same optic on both. I also like how the "K" is the same width as the CZ75 slide. It does not hang over the sides and looks better, IMO. Another advantage of the K is it has a built in rear sight that lines up perfectly with the OEM front sight.