You have to be discerning yourself when reading reviews as people are promoting the product they are talking about. For sigma 150-600 I saw one review that said it had focus "issue" in low light but that is to be expected for all such lenses and camera bodies. All the rest did not mention but said sigma was sharp. Sharpness cannot be measured but is a subjective/ perception matter. It depends on the contrast of subject and background effect.
I'm not implying sigma is bad. Here is one review with objective testing but know that testing with a 2d geometric target is not the same as shooting real life nature in 3d and different lighting conditions.
Sigma 150-600 contemporary:
Link to follow...
Sigma 150-600 contemporary:
That said, you might expect using a camera brand's native lenses will give better results and performance. The Nikon 200-500 f5.6 is a constant aperture vs sigma 150-600 variable aperture f6.3 at the longest end. With the crop factor if 1.5 nikon 200- 500 longest reach is effectively 750 compared to sigma's 600= 950.
Now while a longer reach is preferable it will be harder for a beginner to control image stabaliser not withstanding. Although some can't stand noise in their photos then it depends on the the image stabilisation capability of their camera, lens or both.
I know people will say otherwise but gear does matter according to one nature expert especially to a beginner because if you cannot get sharp photos you will get discouraged and give up quickly when wildlife is not an easy photogtaphic genre to do. Given your starter d7500 + 200-500. It will get the job done anyhow for best results. While gear matter it does not mean photos will look good. Much knowledge, knowhow and techniques are needed. Experts have different opinions as that's the experience they had. For instance the photography rule of getting sharp photo using the reciprocal of focal length eg. 500th sec for 500mm is valid if subject is stationary. Reality is both you, subject and camera are moving so 2X or 3X speed is needed. Although there is image stabalisation, fast shutter speed guarantee less blurry photos . Do not be afraid to shoot at high ISO as getting the shot is better than not getting it.
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D7500 vs d500. Note that d7500 has same sensor and image processor but not the autofocus capability .
Why shoot at F5.6 for telephoto:
EDIT: Apologies for Charles N video as he uses FX fullframe version of 200_500. Below review is a review by Steve Perry who does not waste time in small talk but gets right into the review. For those interested in sharpness at 16.27. Still I'm not sure if he is talking about Dx crop sensor version.If it is then it is just 500mm at it's longest do take note. Apparently Nikon 200-500 f5.6 is a fullframe lens!
Edit 2: After reading more reviews and taking other important considerations which I think you still should be aware of and be practical. The choice of which lens is ultimately important. Don't buy a lens which later turn out to be disadvantageous because of reach ( filling the frame with subject) or disappointment with image sharpness and lastly ease of movement or portability like using a cheaper, lighter or shorter lens like 100-400mm which on your crop sensor will still be 600mm. Think about what you are likely to shoot eg. Small birds which are wary of people close by which mean a longer lens 150-600mm. Or ease of handling 100-400 much like a walkabout lens that you have to carry. The lenses you shortlisted are handholdable but still it will wear you down..
Building up the skills will make using such long lenses easier in the long run rather than making a hasty decision. Please think and decide carefully and you will enjoy what you do.
