My computer:
CPU: Intel i7 3.5Ghz
GPU: Nvidia GTX 1080
OS: Windows 10 64bit
Times - GPU
512x512 1:05.03
256x256 1:05.92
128x128 1:22.09
64x64 2:47.99
Times - CPU
128x128 4:59.63
64x64 4:46.55
32x32 4:40.33
16x16 4:39.29
Result: The GPU is considerably faster on my rig - about 1/4th of the time comparing the best GPU/CPU render times.