Which lens between the two? The Sigma 70-200 f/2.8 is around $800, the Nikon 80-200 f/2.8 is around $900, I haven't shot with either yet. Sigma is an HSM so supposedly focuses better/faster/quieter. Nikon doesn't have that, it's not quite as wide and slightly more expensive but it is a Nikon. Thoughts? Experience?