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

but rather (unacceptable) slow.

What most forget is that you start at f/5.6 with the 70-300 and the 100-400 at 300 mm - so adding a 1.4x or 2x is getting you to an unacceptable AF speed (if it works at all)

But without the 1.4x and 2x extender the investment in a 300+ lens is not suitable for traveling - at least for me - I really like the idea of a good 600 f/5.6 lens. The current cabin restrictions speak agains a 600 f/4.0 setup.

If you stay at your local turf exclusively things might be different though.

You prefer to have a f/2.8 or f/4.0 lens over a slow zoom to avoid being slow even though you've got a good camera.

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