I didn't recomend the 500mm II because of IQ but because of lighter weight, faster focusing and the biggest of all wider aperture and the longer focal lengths.

But thats OK if you want the 200-400mm, I would not sell anything right now untill the 200-400mm is actually relleased and in the stores. It might never come out and it's better to have pics with what you have then whith something you don't.