There are two aspects of RotB prime I like more than BB: I like the shoulders, and I like the truck grill with Autobot logo underneath the windows.
Everywhere else, I'm a bigger fan of BB design. I really liked the BB head especially, which is a little different in RotB.
It's interesting that they seem to have rebuilt the entire thing from scratch to resemble the BB design. Perhaps because they switched to a different effects house they had to build their own new assets.
Let's be honest though we're splitting hairs, they're both great. We're lucky to be seeing designs like this now.