I don't have the benefit of having upgraded from old to the Beta to test the difference - but I will say this : I have the GTX780, and it runs 3 x 24" 1080p screens
smoothly in the Hangar. VERY NICE. When I switch to a Free or G Sync monitor, the results will likely be even better!
I am glad I went Nvidia, though I was dead set AMD at the time of Mantle release. Glad I waited, I think I made the right choice.
Mantle support is great to get AMD on board - good marketing on CIG's part, but trust me - they will quite cynically support and raise the Nvidia flag any day now.