i am thinking of changing my nikon 70-300 vr for the sigma 120-400 os hsm. i use the nikon d7000 and d90. is it worth me upgrading, or should i stick with the nikon and crop the image? i would like a bit more reach, plus the tripod collar on the sigma would be handy. i was hoping that there would not be too much of a loss of sharpness with the sigma.