I'll concede that Peyton Manning is in his prime and Dilfer is not. However, there is more to consider than that. Manning has only played for Indianapolis, a system that features him and Harrison. It's kinda like Clinton Portis going into Washington, nobody knew how he'd pan out in a different system. Not to mention in a different environment. Manning doesn't do well whenever weather becomes a factor. The great ones like Tom Brady and Donovan McNabb can play very well in both while Manning struggles. He may win games but his performance becomes precarious.
The QB's that play well in any environment, not just a dome, will always have the edge. Environments change the perception of the game and how well you perform.
If Manning and Dilfer were both at their prime, I'd probably take Dilfer over Manning just because he has the edge in playoff and weather experience.