Just caught up with the thread. I've found the autofocus on my D7100 to be very good for bif, certainly on a par with my d700 and a different league to my D7000. The af performance is stellar with all my nikon lenses and that is the biggest issue with my sigma, when the light is flat, low contrast or poor so is the af performance of the 120-300, more than acceptable though in good light. It was pretty much the same with my 150-500. I never get that issue with my nikon lenses, even the 70-300 VR blinds any sigma I've ever had for af performance, optically the primes outperform the zooms but not by that much, both sigma zooms I have had ( still have in the case of the 120-300) have been very good. I think that sums it up for me, when conditions are far from perfect I need the better performance of the nikkors, in good light you'll do perfectly fine with any of them, in fact at f8 they're all good.