Ring and pinions are matched to each other and should not be intermixed. You cann't buy them individually anyway, only in pairs. I have setup both new and used and have seen no difference in difficulty that I've seen getting a pattern. Used is sometimes easier as if they are Dana gears they will have a "+" or "-" mark on them which helps set pinion depth. However reusing a used set tends to make more noise as they are never setup the same way they were originally.