The C Series is a mid-level optic, while the B Series is a high performance, elite-level optic…
B Series: Contrast, edge to edge clarity, resolution, light transmission, larger sweet spot – across the board, the B Series has a ~5-10% increase in performance. The internal optical system uses a higher grade of glass throughout the entire unit. All glass is produced in Japan. Units are produced individually and undergo a more rigorous inspection. Overall, they are larger and weightier with a magnesium frame and feel more substantial in the hands.  These compete against the best optics in the market.
C Series: Contrast, edge to edge clarity, resolution, light transmission, smaller sweet spot – across the board, the C Series has a slight decrease in performance, though still exceptional. This will be most noticeable when spending longer time behind the glass. The internal optical system uses a mix of glass grades and coatings. While the B Series are produced and built individually, the C Series is produced in the hundreds, and are sample inspected, rather than individually inspected. The polymer frame makes the C Series lighter than the B Series.
Specific features can be viewed on the individual product pages.