They are quite similar.. IMHO the Sigma, based on a couple of days shooting with only one copy, was quite good but has a very rapid and inelegant falloff to severe blur in the full frame far corners. Mine was only slightly decentered. It had a very nice look. The Nikkor, I had several copies and they were all asymmetrical even after repair - but it was still a fairly good lens. If it were me I'd go for the 24-70 f 2.8 because I've tried them all and it's the best, but it is also a beast. Otherwise I'm with Jack: the 24-120 is so useful, and generally pretty sharp, that eve though it can't meet the demands of the sensor across the frame, it is still good enough and often really good, and for the price and weight it's a very reasonable choice.