I hope you know that the 17-70 is not constant f2.8. From your postings, i realised that you are looking for f2.8 lens only, without preference of any range. Any reason? If you are not sure of the reason, then I suggest you shoot with what you have now, discover what you like to shoot and any limitation of your current lens, then upgrade according to what you need if necessary. no point to just whack any f2.8 lens and buy for the sake of buying.
Sharpness on par, macro useful and works well, minimum focus distance almost front filter touching subject. Slow AF compared to AFS. Sigma has better CA control. Has more solid feel than nikkor. Color is warmer than nikkor. Not suitable for IR as got hotspot problem.