What you're essentially saying, is that Kerry Collins is a better football player than Troy Polamalu.
The importance of your role on a team has nothing to do with how good a football player you are. Brian Westbrook's role didn't exist until his greatness created it. It's not the fact that he plays a larger role that makes him a better player than Jason Avant. Rather, it's the fact that he's a far superior player that makes his role so much bigger. And it doesn't always add up that way. Often times the role is just bigger regardless of who occupies it. Realizing this, it becomes obvious that the position you play has nothing to do with how good a player you actually are.
If you think Tim Tebow is the best player in the nation, it ought to be just that simple. That he's simply the best in your eyes. When you explain your reasoning by pointing out the obvious fact that QB's are more valuable than safeties, you make the game all about the QB's. It's not just Tebow > Berry, but Canfield > Mays, too.
Aside from all that, the original poster went out of his way to clarify this for us ahead of time by including this within his question-