Re: Which is more rare?
Shutdown CB's don't usually last as long as your franchise QB types. Lose a step and your "shutdown" days are pretty much over, Nnamdi Asomugha seems like a pretty good example of this.
I think the term elite gets thrown around a lot for QB's and I think it applies a lot less than what is bandied about. Manning and Brady are definitely elite but you don't really see elite guys at CB that are Manning or Brady's age. Usually they tend to slide over to being a really good safety.
I think with LT's it just seems like very few of them last a long time being considered elite. Walter Jones and Jonathan Ogden are the guys I see as being elite then and I think Joe Thomas is probably the consensus elite LT year in and year out elite LT right now with Clady (injury issues) and Brown (down year) being right on the outside of that elite bubble.
Harder to find has to be QB since every team is looking to find those guys as rookies. They make extremely stupid draft day decisions trying to find that guy, if you have it almost doesn't matter if the rest of the team is just average, they are almost always good for at least a playoff spot if not multiple CCG appearances.