80-Plus Bronze vs 80-Plus Gold

I'm looking to get either the EVGA 850W B2 or G3. The B2 is more in my price range. But is the Gold worth it in efficiency? Aren't they both made by Super Flower, which is one of the best right? Would I be able to run the same amount of GPUs on either? I have an RX 470 and RX 480 at the moment, but would like to be able to add at least 2 more GPUs at some point.

*Would a 750W be okay with 4GPUs (RX480/470) or do I need the 850W?

Thanks