To the extent that karma is an incentive at all, I'd rather have it encourage more high-quality submissions than more high-quality comments. (I'm reasonably happy with the level of discussion in the comments, but there aren't as many interesting submissions as I want.)
I would guess that the supply of high-quality submissions is fairly inelastic compared to the supply of high-quality comments; i.e., there are enough people submitting articles that the good ones will likely get submitted either way, while the same may not be true for comments.
