Just curious: total assumption on my part, but shouldn't the AMDs be better suited to each other? So if you're set on one AMD component for some reason, then the other one should be a 'tighter fit' if also from AMD.
The only real big difference is what tech a game adds. Some add AMD supported tech over Nvidia and vice versa but as far as a CPU and GPU working together, doesnt matter but if i were using an AMD CPU id probalby just go with an AMD gpu for maxim efficiency. Now if youre talking AMD freesync monitors being used with nvidia and vice versa, youll see a bigger difference imo.