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I did carefully consider this lens, but ended up getting a Nikon 80-400mm AFS. This is a great lens and is excellent on the D7100. True, it only goes to 400mm and not 600mm, but since I don't photo small birds I am happy with the more compact size and slightly faster f5.6.
Kent in SD
The weight difference is always going to be there. But as far as the aperture and even image quality, the only fair comparison is at 400mm, where the Tamron matches the Nikon with f/5.6 and probably similar IQ as well. With that being equal, you are trading the extra weight for an extra 200mm. There's still a decision to be made, but it's a smaller decision.