Well if you need a great zoom for regular tele-focal lengths, 70-200mm f/2.8 L II IS is the way to go. It works extremely well for portraits and stage shows. But it you need 400mm buy this zoom. You can attach a 2x extender to the 70-200mm, but that is nowhere close to what the 70-200mm does all by itself. I have tried it, and found the 70-200mm f/2.8 L II +2x extender combination to be a long, heavy, not so fast and not so great zoom. The 1.4x extender is more tolerable on most long lenses.
In fact, if you have a camera that allows f/8 autofocus, the 100-400L+1.4x can be used too. That may work as well on the 400 f/5.6 prime, if you keep the shutter speeds high. Unfortunately, Canon does not have any affordable lenses beyond the 400mm range, so we have no other options.