Canon 70-200mm F4 L with 1.4x converter or 100-400 (1 Viewer)

This has probably been done to death now but as the title says, which would be the better combo?
It will be very much a second system behind my 400 prime but the flexibility would be nice to have.
Just wondering how the F4 non IS L holds up with a 1.4 converter? Mk iii Canon the best I assume?
Thanks in advance.

Hey, I'm not sure I would compare the two options you have listed. I think a 70-200 2.8+2x TC has a closer similarity to the 100-400. That said, I own the 70-200:4 non IS and have managed some pretty good shots with a non reporting Kenko 2x on my old 1D. I know that's not exactly the information you're looking for, but if it helps I'm glad to share.

Without knowing what your kit comprises of, I can throw some quick and dirty recommendations. If I owned a 400 1:4 or 400 2.8 I'd get the 100-400 to have as a flexible backup on a second body. If I owned a 400 5.6 I don't think it's worth getting the 100-400 because you don't gain anything at the 400 end. In that case I'd look at any 70-200 that my budget allows. The 70-200:4 non IS, as has been said before by countless users, is probably the best bang for buck lens in that range. Super sharp, very contrasty, fast and accurate focusing, and best of all very light weight.

I very occasionally use 70-200 f4 non is with a mkiii and have been pleased with the results. Would post one if could find but no idea where any would be amongst my images. Will have a look later today.

Phil