From personal experience, I would say nVidia. I have also been told great things from Kyle at All American Computers about the x3200 chipset from ATI. He absolutly loves that chipset. But you cannot go wrong with either and I am only recommending nVidia because its what I use and I have had 0 problems with it. Ever.
"It is far better to grasp the universe as it really is than to persist in delusion, however satisfying and reassuring."
- Carl Sagan
Intel i7-5960X | ASUS X99-Deluxe | 16GB Crucial DDR4 | Intel 730 240GB SSD | Crucial M4 256GB SSD
WD 1TB Black x1 | 2 x ASUS Strix GTX 970 SLI | Corsair H110 Water Cooler
Corsair 750D | Windows 8.1 x64 | Dell 2410 x 3 @ 5760x1200
Intel E5-2660v1 x 2 | ASUS Z9PE-D8 WS | 128GB ECC DDR3 | WD 250GB | 24TB iSCSI Shared Storage