I've owned both the CF and the CB.
- The CB is slighty less sharp, but you need good eyes and high-res film to see & appreciate it.
- The CB has visibly more distortion.
- The CB has better colour saturation.
- The CB has a much smoother focusing ring.
- The CB is slightly 'fatter' (more difficult to get to the release button).
Because of the distortion I sold the CB and kept the CF. But the differences are still minimal.