Unless you plan to shoot wildlife, need the extra low light, or want to use Canon extenders, stick with the F4. The 5DIII can easily make up the light loss by using a higher ISO. Many people think the F/4 is sharper than the F2.8II. It's definately sharper than the F2.8IS but likely just comparable with the F2.8II.
That being said. I often use my 2.8L II with the canon 1.4x and sometimes with the 2X so you get more flexibility with that one but like I said, if you dont really have the need for it, save the money for when the 7D MarkII comes out.