I'd probably go for Colette but honestly neither.
My friend is a Clementine (Clemmy) and extremely beautiful and cool. Still the name Clemmy makes me think of phlegmy and clammy. If she can't carry it off I can't imagine who could!
Colette sounds like an old lady and not in a good way.
I like Cosette as suggested by a PP. Or if we're going with the C theme what about Carla?