There are some similarities and differences between these two denoisers.
Most of all, the classic denoiser can be used in animations (provided the based image quality is good enough), and the GPU denoiser will most likely cause splotches/artifacts jumping from frame to frame.
Another thing is that the GPU denoiser is "progressive", so you can just keep rendering, and stop once you are happy.
With the CPU denoiser you have to render an image, wait for the denoising, check if the quality is good enough, and either accept it or re-render (or resume rendering).
So far we don't have exact numbers like "use 5% for GPU denoiser and 3% for CPU denoiser", and this is definitely scene-dependent.