Can't comment on the Sigma, but the G versions of the 24-85mm f/3.5-4.5 is a great performer. Much better than the AF-D version that should be avoided. It is not weather sealed nor does it have the Nano Crystal coat of the gold ring 24-70mm f/2.8 or 24-120mm f/4 VR, but from an optical standpoint, once stopped down it gets very good.
Other than high end special items, I've never had great luck with 3rd party lenses. I've heard that Sigma and even Tamron stuff has gotten much better recently. That is certainly a plus. But it is relatively easy to build a kit of lower cost Nikon zooms that performs very well like:
18-35 f/3.5-4.5G
24-84mm f/3.5-4.5G
70-300mm f/4.5-5.6G VR