Blender 2.79
My Time
CPU:E5-1620 v3, 3.5 GHz
GPU:GTX 970, 1114MHz
GPU
x=128 y=128, 1m49.93
x=960 y=540, 1m19.49
CPU
x=128 y=128, 3m58.75
x=960 y=540, 21m16.98
There is one thing that catch my eye.
Both GPU and CPU render results are identical regardless the tiles size.
But the results between CPU and GPU are different around the the transparent head light cover. the noise patterns are slightly different.
The one thread CPU render (x=960 y=540) takes pretty long time,
but the task manager shown that no CPU is in full loading,
After Path tracing tile pass the point 200/400,
I could see no differences except the head light bloom.
Conclusion, I think I will stick at GPU rendering.