I have used both the GR4 and the B7. I've gone through two GR4s and currently on my first B7. While the comfort isn't as good as the GR4 the traction, support, and durability are there.
Although I still prefer the GR4 if I had to pick one.
Strong is what you have left when you've used up all your weak.
Faith and Perseverance. Yonex Vcore 97 Tour (330g)