After a good 30 minutes or so going over every millimetre of the lenses I finally decided on the slightly more expensive one as it had flawless glass. Looking through the barral of the lens I didn't see any fogging, dust specks, scratches or mould. Where as the other lens although still pretty great did have some small amounts of dust on the inner elements. This wouldn't show up in the images of course but it wouldn't be a good thing to have if I ever wanted to resell the lens in the future.
I took the new lens out today for a little bit of a play and test run. It performed beautifully as it should. And I was surprised to see the quality was still very impressive when combined with the Canon 2x mkII teleconverter. I was not overly impressed with the image quality of the 2x extender when combined with my Canon 70-200mm f2.8L but with the 300mm it was just superb! Autofocus of course does slow quite a bit when using the 2x but its still quite useful.
Here are some sample shots I took with and without the 2x tc on the 300mm. Please click the thumbnail to view the full size image.