I am beggining to take pictures of birds. I do not have the money for the big primes. My camera is a D700. Which lense would you choose? thanks a lot. Nikon 80-400mm f/4.5-5.6D VR vs Sigma 50-500mm f/4-6.3 EX DG APO HSM Telephoto Zoom Lens for Nikon SLR Cameras.