I disagree and I know I'll probably get flamed for this but I really think that Grant is better than Smith. Grant can play both end postions and move to the inside if needed and he can be effective at all of those postions. Smith can only play at one postion on the line as an edge rusher.
I compare Grant vs Smith to Rickey Jackson vs Pat Swilling. Swilling would get more sacks because he was rushing from the blind side, but Rickey to me was the better of the two despite him getting less sacks.
I do believe Grant is very much underrated. Some of his contributions aren't quantified statistically, much like an offensive lineman. I think it is mistake to compare Grant and Smith. It is almost like apples to oranges. They play the same position, but have very different jobs and one benefits from the other. I do know the every time Grant was not on the field, the defense was horrible. That said, I think we overpaid for Grant; though I am not sure we had much of a choice. I think Will Smith is as good if not better than Grant, and would pay him more if we didn't already pay Grant so much.