Here are my results, typically my gpu did better across the board.
512x512
GPU:1:21
CPU:13:22 (estimation at render being half that time at 50% complete, i didnt have time to wait lol)
128x128
GPU:1:49
CPU:5:33
I was pressed for time and could only do those, but id say even with 8 threads the gpu is just the better choice