It all starts up front, Rich. I would argue that with the addition of Ngata, Ray Lewis has seen a quite the rejuvenation in his career. Sure, he plays with a good supporting cast, but he is a very, very disruptive player in his own right. His presence would surely make players like Kelsay and Poz more effective. That's not to say that the Bills D would become as fierce as that of the Ravens, but the impact would be significant. Ngata is the type of impact player you take in the Top 10.
Contrast that to Whitner, whose main impact has been his playoff guarantee and Thursday night movie nights at his house...
I'll take Ngata