I had a C17 on a Surly Pacer several years ago. Sold it with the bike. I now have a C15 on my commuter and a C13 on my race bike. So..
C17 – widest. More of a touring saddle width really but very comfortable. If its for mountain bike or more upright type road riding then this would probably work well.
C15 and C13 – available in several widths. From memory Brooks advises going 1-2cm wider at the saddle than your sit bone measure as the saddle is more of a hammock than a perch. I have 145mm from memory. Both come in a ‘carved’ option which has a central cut out to save your perineum. The C13 is lighter as the frame is carbon. It has oval carbon rails and is more expensive than the C15. I find my C13 also feels firmer than my C15 (although that could be because the C15 is a carved version and the C13 is not).
C15 feels more comfortable to me but at a weight cost.
HTH?