B2 is semi, G2 is fully modular. B2 is Golden Green Super Flower platform, G2 is Super Flower Leadex platform. G2 is a higher quality unit, but both are good power supplies. If you want the better unit or full modularity is important to you, then go with the G2. If price is all that matters so long as you're still getting a pretty good unit, then go with the B2. I think Newegg has a shellshocker deal today on the 750w G2 for 89.99 after rebate.