Depends on who the two players are. A young Donavan Tate who went In the top 5 picks, a young Bubba Starling or a much older Buster Posey or a much older Pedroia. Who had the most upside?
IMO, all things being equal simply doesn't exist. Problem is it is difficult to know for sure who will end up being the best. Being one year younger can be an advantage, then again sometimes maturity can actually be an advantage.
If one year made that much difference the entire first round would be high school kids every year. Why do they draft so many in the first round that are, not one year, but three or four years older? If only it were easy to know which player, one year apart, has the most upside