For field sports action shots, personally I find the Sigma 100-300 F/4 more suitable. Imagine to be able to frame a player further away one moment and the next swivel around to capture a decisive moment of another player feet away from you with the zoom.
Yup thats why I'm taking the Sigma 100-300 into consideration. I tried shooting with a 70-200 F2.8 last time, its just too short when 50% of the time all the action is happening on the other side of the field. When the action comes nearer, having a zoom is more flexible.