For me, it's all about the best player available (BPA) that fills a need for your team. The only exception is QB because the NFL is a QB driven league and you need one to win it all. There are a few exceptions like when you have a dominating defense like Denver did.
Summary: Patrick Peterson is one of the top prospects to come along at the cornerback position in years. His size led some to believe he should move to safety. While he has that ability, he clearly has the speed to stay at corner - which he will. Like the legendary Deion Sanders, he can hurt the opponent on all four downs after showing what he can do on punt returns in his final season at LSU. As with many freak athletes, he needs to rely less on that ability and be more disciplined.