Both are excellent lenses. However keep in mind the F4 IS has only 1 or 2 stops of IS correction. The 2.8L II has 4 stops!!! You are not only getting faster shutter speed at F2.8 but you can actually hand hold up to 4 stops less than that.
I've used both and now have the 2.8 II. It's by far the better lens technically (sharpness about the same). Plus you can use the 1.4 and 2X extenders with the 2.8 if needed and still have a very useable 300mm f/4 and 400mm f5.6.
Agreed. I had owned the F4, but if you have the funds definately go with the 2.8II. Fast and sharp.