The significant difference between the breeds is the length of their tails.
They will perform much the same in the uplands with a greater difference between individual dogs than breeds.
They will handle waterfowl equally too with differences between dogs more significant than breeds. I don't consider either to be candidates for big water divers but for the overwhelming number of combo Hunter's either will be fine.
GSPs can be easily found but that also means one has to research the backgrounds better to get a decent chance at a good dog. Vizslas are less common and the breeders tend to be more discriminating as a whole. The three Vizsla breeders I know have waiting lists in excess of 3 years after the prospective owner successfully concluded a questionnaire and interview.
For either breed, I think the North American Versatile Hunting Dog club would be a good place to start and likely finish your search. They have a pretty large registry and their testing is geared towards the Hunter for the most part.
