If you are a smart team (and by smart, you are willing to pay the bonus demands), you can find guys who have high upside and are safe bets to reach it. Guys like Bruce and Maybin have high upside, but were pretty safe bets to reach there potenial. If you mean if is best to draft high upside highschool guys or finished product college players, then i think it is best to take the player avialable of either group, like the red sox do. They take the best player avialable (no matter the signability questions), which in 2005 was craig Hansen and Jacoby Ellsbury, who were close to ready college prospects. In 2006 they took a couple of unfished by high upside guys, in Jason Place and Dan Bard.
"I do not think baseball of today is any better than it was 30 years ago... I still think Radbourne is the greatest of the pitchers." John Sullivan 1914-Old athletes never change.