There are several threads already covering those lenses.

@ry_ka - for what it's worth, the owners of the privatly owned camera store that I occasionally visit say the Sigma 70-200mm EX DG AOP OS HSM II lens is a good one. They're Nikon shooters also. There might be a sprinkle of "sales pitch" in what they say but at over $1000 less than the Nikon it might be at least worth looking into. It's not weather sealed as the Nikon brand is.

There are 12 reviews on the Amazon site and about the same on the B&H site and most are positive. I'd consider it but currently trying to get the best out of the Nikon 70-300mm VR lens that I currently own before I spend more money. Good luck.

I have the 80-200 2.8 AFD

It's a fantastic lens! Sigma costs about the Nikon. What Sigma has going for it is the vibration reduction which the Nikon doesn't have. If you are willing to forego some sharpness and build quality of the Nikon, I would go with the Sigma.

Keep in mind that Sigma has quality control issues so you might have to check out a few of them before finding a good one.

hi guys...im planning to buy a telephoto lens for sports, festival(street dancing), concerts and bike race(or other)...im not a pro photographer but a serious hobbyist..

my choices are:
sigma 70-200 ex dg apo hsm II
nikon 80-200 AFD (2 ring)

my camera body is nikon D90...i can't afford the nikon 70-200 vr2.:)
my concern to the sigma lens are the negative issues and reviews like front/back focusing problems, sometimes AF stuck, peeling of the lens skin and the image quality.. well, there's no question about the build and the image quality of the nikon, my only concern is the AF speed of the lens in my D90..

any comments and suggestions?