The fundamental difference is the type of prisms used in each. The B.1 uses a traditional Schmidt-Pechan roof prism and the B.2 uses the Abbe-Koenig prism. Due to the prism size the B.2 is slightly larger and heavier, but also offers 3% more end-to-end light transmission. The B.1 is slightly more compact and comes in the traditional 8×42 and 10×42 while the B.2 is slightly larger and comes in 9×45 and 11×45. Generally speaking, the larger 45mm will gather more light and will perform better in lower light conditions.