Hi we are keen amateur photographers on a budget and would like to expand our horizons with an upgrade in lenses, we are thinking of a 24 - 70 f2.8 and a 70 - 200 f2.8. Is the difference in price between nikkor lenses and say sigma / tamron (maybe around half the price) that significant in the quality of the final product ie the photos and or the usability and build quality. We currently own a D300s and a D90 which we hope to upgrade to a full frame in the future. we would appreciate your expertise.