- I have a 70-200 f/2.8L IS MkI, would adding an extender to this be just as good?
- How about the 100-400 L? It gives more reach, but the IS and AF are old vs the 70-300L
The IQ of the 70-200/2.8 MkI + 1.4x will fall short of the 70-300L - substantially, in fact. Also, the TC will slow down the AF. The 70-200/2.8 MkII + 1.4x would come quite close to the 70-300L for IQ, but at a much higher cost. But, with a 70-200/2.8 + 1.4x, you'd have one more stop of light at the long end. Still, I'd not recommend planning to routinely use a TC, but rather, get the focal length you need.
The 100-400 isn't as sharp as the 70-300L; the AF is quite fast, and the less effective IS will likely not be a huge deal as you'll be shooting mostly at faster shutter speeds anyway, I would think.
Really, I think the choice should be between the 70-300L and the 100-400, and that choice mainly comes down to whether or not you need the extra 100mm. If you do, the 100-400 is the way to go, and if not, the 70-300L is a better choice.