I recently (12 month ago) had a similar issue.
I do own an 70-200 2.8 IS MKII.
I do NOT own a 2x-konverter
After some rental of the 100-400 I decided to go for the 100-400 instead of the 2x-konverter for the following reasons:
- quality of the 100-400 better then the 70-200 + 2x kombination
>>>please make shure that you've checked the site:<<<
- price (in germany, 1 EUro = 1.3 USD approx):
2x-konverter only: 500 Euro / 100-400 optics: 1.400 EUR
of cause a big difference, but at the 100-400 you get a complete optics which you'll be able to sell for some money as well later.
- handling: i personally do not feel comfortable to frequently change the lenses on location, the converter adds some chances to get dust in the system.
the 100-400 is a affordable (somekind) good optics which deliveres excellent results.
the 70-200 + 2x is a good workaround which deliveres good results.
Please keep in mind that automatic lens-correctsions (dxo-software, pp) might not be available for the 70-200 +2x-kombination.
My recommendation is the 100-400, if you already do own a 70-200.
if not the 70-200 of cause is first.
even then in my eyes the 100-400 is the better invenstment.
but of cause, it's the money that counts.
Hmmm... I'm a big (you can read in fat since I'm american although not too fat by american standards), strong and very tall. I photgraph mostly birds because there's not a whole ton else interesting where I live (central connecticut). Therefore I find it easy to handhold big telephotos and run through the woods like a crazy person for the perfect light angle. My tripod stays on my pack a lot and if it can be restrictive for the running around like an a-hole part of my process. I've researched the lens comparisons you referenced many times and the IQ drop is not real important to me (relatively small) compared to the better IS but the AF accuracy is a big deal. I'm getting close to purchasing my medium telephoto lens/lens combo and this question has been on my mind a hell of a lot. The pros seem to be picking the 70-200 with 2x. This will not be my primary lens and I love how the 70-200 looks without a converter and that's huge part of my current thinking and although I am a big guy I'm also psychic and I see a back problem in my future especially because of the 600mm. Given that you've done the research is there anything you can help me out with?