Its tricky, because the common sense is that the IS isn't going to do you any good for sports since people are moving so fast...I somewhat disagree with that, because the difference between f/2.8 and f/4 is the same as just bumping up your ISO one stop, and unless you're already at the upper limit of your camera, or the upper limit of usable image quality, I think the f/4 IS is definitely a good sports lens. Additionally, sometimes you don't want 1/250+ action-stopping shutter speed, and in that case the panning mode with the f/4 IS is more valuable than the f/2.8 non-IS. The idea that only f/2.8 lenses are professional is becoming less true every year with the advances in digital technology. In film days ISO 1600 looked pretty bad, now you can't buy a DSLR camera with IQ as bad as even the best ISO 1600 film (and the difference is probably a stop or two better).
You can't think just in terms of exposure for sports. You need to think in terms of types of AF sensors that are accessable to the camera for focusing. An f/2.8 lens MIGHT be able to use dual cross types, for instance, whereas a partcular f/4 lens cannot. This is exactly why I have 300 f/2.8L vs. the 300 f/4L. High action focusing in low light.