B+ is an amazing variety to work with, the spores are super resilient and just love to fruit.. I've been using the same B+ spore line for 10 years.. I love them .
GT have a more ryzomorphic growth(quick and veiny in look rather than puffy and slow growing[keep in mind that doesn't mean it'll be ready any faster]) and they usually produce shorter fruits that tend to cluster
Both great varieties but as all mushrooms, it's environment and genetics on those monster fruits