I highly recommend going with the 2.8 since you are shooting sports. I know the 5D MKIII has great ISO performance, but you would be surprised how quickly you will be increasing the ISO as the light starts to dim. For example, tonight I went out the lake to take some photos of birds and other wild life about 3 hours before sunset. While shooting under the trees, I was at ISO 400 at F/2.8. When I switched lenses and was limited to F/5.6 in the shadier spots, I was at ISO6400 to get a shutter speed of 1/500. Sure, you can shoot at that ISO on the MKIII, but you still have some noise and loss of detail. The IS won't help you at all for shooting sports, but the one stop in aperture could come in handy.
I tried to decide between the same two lenses about two years ago and went with the 2.8 and do not regret my decision.