For sports photography, should I use image stabilization or a faster lens?

For sports, I know that a faster lens would usually be considered more important than image stabilization, but what if, after selecting your shutter speed, your camera is picking apertures that don't require a super-fast lens?

Here's my specific example: I bought a Sigma 70-200mm F2.8 lens for my D90 thinking that I would need the F2.8 in order to shoot at my daughter's Figure Skating club. I chose the Sigma over the Nikon 80-200mm F2.8 because the Sigma had full-time manual-focus and a built in auto-focus motor and was about $100 less expensive. The Nikon 70-200mm F2.8 with VR was well beyond my budget.

Now, having used the Sigma a few times, I am pretty happy with the shots I am getting. I set the camera for a shutter speed of 1/500s and ISO-800 and let the camera pick the aperture. However, it turns out that most of the shots I am taking end up with the camera choosing an aperture of F4 to F5.6 (the lighting in the rink must be better than I thought).

So, my question is, would I be better off returning the Sigma 70-200mm, and getting the much less expensive Nikon 70-300mm with VR (saving almost $500 that I could put into another lens or a tripod). The aperture range of 4.5-5.6 on the 70-300m is around what I am shooting at, plus the lens has VR (I know it doesn't help with moving subjects, but I am shooting hand held at 1/500s for the moment).

I could even bump the ISO up to 1600 to maintain or increase the shutter speed with the 70-300mm. On the other hand, in a different arena, the lighting might not be as good, and I'll be wishing I still had the F2.8.

Answer

Generally good advice with regard to IS and it's ability to freeze action. However I'm surprised no-one has mentioned the fact that some camera bodies have extra sensitive AF points that are only active with f/2.8 lenses. Thus if you have an f/2.8 lens, even if you end up shooting at f/4 or f/5.6 you are gaining an advantage from the max aperture in terms of focussing performance. This is why f/2.8 is the holy grail of sports lenses, it's not for the speed, it's the for the AF.