Do you "need" the thinner DoF of the 2.8? That can be a big factor that the f/4 lenses can't do.


Do you need the action-stopping of the 2.8? That can be a big factor as well.


When shooting still objects, your 70-200/4IS has about the same shutter speed/camera shake limits as the 70-200/2.8IS, due to differences in the IS units. For people, that stop of aperture can really change the landscape.