I think sultanas are like baby slugs (yeuch), will make them with currants (well, I will buy the stuff and um and ah about it and maybe make them some time in the future).
does it matter, one being dried grapes and the other dried currants like red or black....I guess it is a taste thing, so sherrard, whatever you like or what you have at hand. From what I know of sheep food, both are fully acceptable....