Had the sigma for a couple of years (non OS version). Found it to be excellent with a 1.4 tele fitted and you just get used to the weight.
However after checking what lengths I normally used it at - 18,000 pics Over 95% were at 420mm...
So sold it and bought the Nikon 300 prime.
The sigma I'd give 7/10 (with maybe another point for the zoom if that's important to you).
The Nikon 200 - 500 is interesting to me but I can't see the focussing speed being as quick, even though the zoom range is impressive.