That is a very tough question.
I would agree that McNair is injury prone. Isn't he always hurting his ankle and leg? I don't know if it's the same injury over and over. You know sometimes those need time to heal and of course with a game every weekend that's just not enough time.
With Favre I think it's his thumb over and over.
On the surface and off the tip of my tongue I'd say Favre because of the games played consequently.
But there is a mighty good argument for both.