600mm f5.6

Hello all, Has anyone used both the 300mm f2.8 afs and the 600mm f5.6 (or f4)?? Need your wisdom. I like to shoot wildlife and birding and realized that my 70-300vr isnt cutting it as far as sharpness goes. Its actually great at 300mm, but a review of flickr's pics rendered from 300mm f2.8 is...