The 2.8 is what I own. IS is for hand holding the lens at slower shutter speeds, but as you said does nothing to correct subject movement.





2.8 lets in twice as much light as F4.0. This means your shutter speed will be twice as fast, which can mean the difference between capturing action or not.





if you intend to shoot sports, indoors or out, I would advise looking at the faster glass 2.8 it will allow you to achieve higher shutter speeds. thats really what it comes down to, your intended usage.