[JJ_SO]

I neither will carry that 4 kg, but: Only a grand more? Forget the Sigma.*

1. I don't recall I ever used the zoom or needed to zoom. This lens is about using it at 600 mm and
   
2. those 20% more of focal length is in fact overrated: I reckon, even in a blind tets you will prefer the 500 mm crops against what a Sports can deliver.
   
3. One Sports was on sale, before I got mine. His owner was happy with the performance but came to his conclusion, I rather have f/4 at 500 than f/6.3 at 600 mm. For him, that wa sreason enough to bite a bit more than only a grand  -_-
   
4. 4 kg is heavy, yes. But 2.86 kg (Sports) is not that much more on the "freehand"-side.  ^_^
   

* edit: I don't say, go for the Nikkor instead - but I'd say, if it's in good condition and one of those lenses coming just on the market to collect money for their successors and not because it's battered and was used as primary weapon in a debate with a Rhinoceros, you probably are better off with the Nikkor - although Sigma's OS can compensate a lot.