As others have mentioned the 70-200 f4 plus 2x combo will not autofocus on anything but a 1 series body. That is a deal breaker for me. However it is a more modern design with better IS and weather sealing. The 70-200mm f2.8 will autofocus, but it is much more expensive.

How often do you plan to go above 200mm?

I have recently been trying to figure out the best/reasonably economical way to add some reach for wildlife and I think the 100-400 is the way I will end up going. Here's my thought process.

100-400:
Pros: Very versatile focal range without changing gear, reasonable price for 400mm, good IQ over the range, IS
Cons: Relatively slow at the wide end, older 2-stop IS, not completely weather sealed

300mm f4 + 1.4 TC:

Pros: Relatively fast at 300mm, autofocus works on all bodies with the TC attached, good IQ, constant physical length, reasonable price, IS
Cons: Limited to 300mm and 420mm, old 2-stop IS, not completely weather sealed, gear change to get to 420mm

70-200 f2.8 + 2x TC:
Pros: Amazing IQ 70-200, good with TCs attached, up to 4-stop IS, weather sealed, constant physical length, fast
Cons: Expensive, gear change to get to 400mm, IQ at 400mm not as good as the 100-400 see link
http://www.the-digital-picture.com/R...mp=5&APIComp=0