Id usually go for a 60mm scope over any 50mm scope for obvious reasons, but the ED50 seems as good as most 60mm scopes, however the difference would only be noticed around 40x.
I will be using @20-30x EP fixed so i dont think i would notice any difference between the two, and as it stands the Kowa set up is considerably more affordable than the ED50 and with the larger objective it would be more usable in all situations.
Still the little ED50 seems a belter of a scope. The kowa comes with the EP i like, with the Nikon id have to fork out for a 27x EP another (150-180 clams). So im swaying towards the Kowa if i can get it, otherwise i will have another look at the ED50.