For KHC mining, AMD GPUs are 4 to 5 times faster than nVidia GPUs.  This is because the KShake320 algorithm is Hash-based and requires only Integer Arithmetic and Logic operations. The algorithm benefits from the high number of ALUs provided by the AMD architecture.  
nVidia architecture on the other hand, is much stronger on the Floating Point calculations but, the KShake320 algorithm does not use the FPU.