300 f2.8 IS L II plus 2x Mk.III better than 100-400 f4.5/5.6 IS L?

If you are shooting wildlife or sports, need the added reach, don't need the portability of the 100-400, AND are willing to spend the money for the 300, I'd advise getting the 300 and both mk iii extensions. The 300 2.8L IS ii is absolutely spectacular both alone and with the extensions.

The 100-400 is a great lens with excellent IQ and I wouldn't part with it. It's very practical for carrying around and extremely versatile. It seems really light now compared to my 300, especially if I have an extension attached.

It took me a little while to get the 300 dialed (micro adjustments) on my 5D mk iii. Below are some  photos when it was close, but not dialed in all the way:

With the 1.4X



300mm

Michael