Jayson,


I have both the 20-700 f4 and the 70-200 f2.8 IS. ( I also have the 24-105 f4 IS, that I think you should also consider.)


In terms of image quality there is nothing to pick between the f4 and the f2.8, but the 2.8IS is almost three times as expensive.


I use the 2.8 IS indoors (especially churches) where I do often need to go right down to 2.8 with high ISO. Outdoors the advantage of 2.8 is the shallow depth of field, but really the f4 will do just as well most of the time.


I use the 2.8 on professional jobs (when I'm carrying so much gear the extra weight is of no consequence) but I'll often take the f4 when I'm just out taking a camera with me.


In your case I really would suggest considering the 24-105 f4 L IS. This is my normal "walkaround lens" on the 5D when I'm just out with the camera, as distinct from working. It would be perfect for "kids in the yard" and on a crop factor camera quite reasonable, with a bit of cropping, for reasonably close sports like tennis.