As a recent seller of a 70-200 Sigma to a happy forum member, I have a bit of an opinion. Yeah I purchased the VR version. The Sigma is and was a great lens in terms of sharpness and it would be splitting hairs to really differentiate the two lenses.
Value for money I would go for the Sigma as it is half the price. Optically there isnt that much difference although I am sure many would disagree. What makes the real difference to me is the faster focus and the VR. The Nik VR has also a more robust finish ie mostly metal whereas Sigma has this matt softer surface that easily scratches.
I found VR a bit strange at the beginning and it took me some time to get used to using it properly. The focusing is faster and for this focal length this is a real bonus. I hope to keep it for 20 yrs as with most of my other Nik lenses.
Last week I was again in Samoa and attended a Fia Fia (Dancing and singing) I was able to take about 300 photos hand held in low light at 2.8 and got some gems that I attribute to the VR function. Sure you can use a tripod sometimes but most of the time I just want to attach it and walk or stand on a chair in the middle of a open restauarant as I did in Samoa.
I think Sigma is really getting its act together and like all competitors, giving Nikon a good reminder that slow response can lead to changes in loyalty. Hey! Sigma has a 17 or 18 to 70 and IMHO Nikon should realease a 17-70 2.8 to put us all out of our misery.
Where Sigma loses it a bit is with too many new releases of lenses. I like having a few bits of the latest kit and its hard to do with Sigma. When I purchased the Sigma late last year the DG version had just been released and within six months they released a newer version which has even closer focusing. Good things dont need updating as quickly as this.
As life changes, I am more interested in purchasing fewer but better lenses and I am happy with my new Nikon 70-200 but would not for one minute criticise anyone with a Sigma of the same calliber. Its really more about photos than the gear so let the photos do the talking.You need to take at least 3-5000 shots before you get used to your lens.
Just a few thoughts in this interesting discussion.
Peter Mc
PS: the 70-200 Nikon makes my D70 feel like a plastic toy. I used to attach lenses to my camera and now I attach my camera to the lens. I think I need a D200 to keep up with my lens...good excuse hey