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

Gearóid Ó Laoi, Garry Lee wrote:

Compare and contrast..

1. Teleconverter. You you have your 300mm and you say, "hold on a minute, I need a 400 now"

You take off your precious 300mm, look for an area of ground free of animal droppings, put it down carefully. Then you root in your pocket for the teleconverter container. You open it, take it out, remove the caps, put them carefully upon your person, reminding yourself to remember where they are, stick on the teleconverter, then pick up your carefully deposited lens, upon which by the grace of God, some seagull has not evacuated himself, or herself, try to work out where you start the twist, twist and return to your eye.

2. Zoom. Keep at your eye and zoom.

Bit of a difference. This is why, even though I have three teleconverters, I've used them about the same number of times as Italy has won the World Cup...