Thanks for the comparison Larry. Definitely a reason to consider the f4 if your shooting does not require the 2.8, or faster since not all of us shoot sports exclusively with a zoom.
Zone - I love the comment "Next camera generation, you WILL be able to use an f/4 lens instead of the f/2.8 lens (because ISO 25,600 will have the image quality of today's ISO 12,800). But until then, the f/2.8 lens is needed."
So true, but the reality is that fast glass will NEVER go out of style, because in this case you would still rather have a 2.8 vs 4 and be able to shoot at 6400 instead of 12800. Shoot, I'm waiting for 25600 to shoot like 4000 and then walk into the gymnasium and shoot 1/1000th on ISO 200. LOL It's all good. How blessed we are to be able to debate the "issues" of improved performance