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:<<<http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=113&Camera=453&Sample=0&FLI=7&API=0&LensComp=687&CameraComp=453&SampleComp=0&FLIComp=5&APIComp=0
- 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.