Faster will be doing it with the GPU as long as it's a typical system with a far more capable GPU that CPU.
You would have to list models but unless you have a super strong CPU and a very weak GPU, the GPU will be faster.
The problem with using GPU based encoding is what GPUs can do are limited. For the same quality, you're likely to have an inflated file size compared to using a CPU based encoder. I recently did a test with this on a 1080p Blu-Ray rip in Handbrake. The x264 output was half the size of the NVENC one.