[silkstone]

I believe that the Pro version has more elements and better performance especially at the edges. I'd go for the PRO 300 DG as Steve suggested earlier. I have the earlier version of the 1.4x (without the 'DG') and that works on the 70-300 IS although it means that the max aperture at 300mm is f/8 and autofocus can be a bit hit-and-miss unless light levels are fairly good.