It depends ;). Some players value higher younger player with medicore stats and ratings (A- is medicore and means probably player hav lower than 55SS), other want older player with nice stats and chance for beeing 70+ SS (small chance but there is)
I'm as much a fan as anyone of drafting the 19 year olds with decent skill sets to be deep bench players who might pick up random weeks of training when there are injuries or two position weeks, but if you're talking about 5 balls of potential, that's much more valuable on an 18 year old than a 19 year old. In an active league, you could well get both if you put the 18 year old as the first choice, but you never would in the reverse order.