i use the b1 on electric guitar every now and then, its excellent for when the guitar needs more "air" than a dynamic (ska, jazz, slow boring annoying blues). i'm a big fan of the b1 on bass cabs and room mic duties aswell.
i haven't used a c1 before so i am a little unqualified here, but my feelings were that the c1 was a "coloured" sorta vocal mic, and the b1 was a "neutral" kinda all round mic.