I'd have to say experience. Skills may get you by some of the times, but unless you have someone who knows what it is like to be in such a situation, then it is the luck of the draw with skills.
Definitely experience. I have seen talented players break down in key situations because they don't have the experience to make the right decision. Young players that have allot of skill but little experience are often brash and quick to make decisions without seeing what the consequence of that will be. They just haven't been in the clutch enough to see two or three plays/minutes into the game past their current play.