Leonard is definitely the more straight guy of the two, while Sheldon is like his weird sidekick. That said, Sheldon in his own right became very important to the show, to the point that the majority of show's plots would revolve around him. However, it still seemed to be more from Leonard's perspective.

I'm torn, but I lean more towards Leonard. At least right now. You could ask me a different day, and I may say Sheldon.

I think they're part of a long line of character pairs where one is recklessly impulsive, and the other is his or her caretaker. Lucy and Ricky, Ed and Wilbur, Jeannie and Tony. So I think they're literally a set - one is not that interesting without the other.