I had a B17S. the rails on it are very short. I felt like I couldn't get far enough back to feel balanced.
My husband said the rials on the regular B17 were much longer. We sold the B17S and got the B17. The rails were much longer, both fore and aft, and we were able to position the saddle about 1 1/2' further back on the seat post than with the B17s. It helped a LOT my feeling of being off balance.

I responded recently to another thread on this same subject, in which I included photos comparing my two saddles on the same bike:
http://forums.teamestrogen.com/showthread.php?t=37357