With Kelsay the Bills' defense was no great shakes. I'd go with Hargrove and take my chances, at a fraction of the cost, which can be used for more important things, like keeping Nate, getting better OG's, and adding a DT. Kelsay will be SORELY overpaid, just because he's one of the better DE's of a bad FA crop.
As for Willis, he averaged 3.8 YPC, caught 18 passes, and scored 6 TD's. Hardly the stuff of legends.