@xiota - the Sigma is very soft at the long end and from what I have read, and been told, the Nikon is much sharper. Based on the link you have provided, it would seem I have been given bad info. I will find out shortly as the lens is about to arrive...

@MichaelC - very true, but I guess I was just looking for a bit of guidance as to whether my selection was good for my needs :-)

Thanks you for your answer, I should have probably mentioned in my question that I already have a larger lens (Nikon 200-500mm). You are absolutely right, I will be using this larger one a lot more than the one in question, but I want to make sure I purchase the correct one nevertheless.

@null - Sorry, I should have mentioned this in the question - I am not open to renting any lenses as my trip is for 3-4 weeks, and I am likely to be going again later in the year so renting will save little money in the long run

@MichaelC - I don't know that, in fact, I think it is very likely to be my technique as I very much struggle to keep the camera steady, especially when taking pictures from my vehicle. I guess I am looking to purchase lenses that are more forgiving to my shaky hands...

I upgraded my D90 to D500 because I wanted the faster shutter speed, better autofocus and many other superior features of the new camera body. I want to upgrade my lenses because I want to get the most out of my new camera body by having lenses that compliment it. One of the areas that I have struggled with in the past is getting perfectly sharp images and I am hoping that upgrading my lenses will assist me in achieving this. Apologies if I have not explained it well enough beforehand, I am new here so still struggling to get to grips with how to ask the questions etc...

@MichaelC - to put it simply, sharpness. This is the area I struggle most with

@Hueco - let me know your thoughts on this if you get a moment: photo.stackexchange.com/questions/103814/…

@FábioDias - see here: photographylife.com/…

@FábioDias - Yes I did, and as much as I would love the extra 100mm reach, I ideally want to avoid third party lenses. That is not to say they are bad, it is more the fact that Nikon lenses are specifically fine tuned and designed for Nikon cameras, thus they tend to be better. Note - I am a fan of Sigma, most of my old lenses are Sigma :-)