To me the question is a bit odd. The 600mm is made for a totally different use than the 200-400. They are both exceptional lenses and outstanding for their use. When going on a trip, where I don´t bring both, it has not been difficult to choose which one to bring.
It would be more difficult to compare the 200-400 with the 300 f2.8L IS II, with the 1.4xIII and 2xIII extenders. The 300 is clearly less flexible, but it is smaller, it gives you exceptional IQ, AF speed and a stop advantage and also very good performance with the extenders. You also have significant money saved for something else.
It´s a substantial cost to get both the 200-400 and the 600, but it would have to be for something really exceptional if I were to part with any of them.
A agree Eldar, both these Lenses are a substantial investment for most that decide to buy either, or both, and one would imagine a serious amount of thought goes into that decision.
The 600f/4 is not your close in type Lens, it's uses are varied but going on CR and the many fine Images I've seen with the 600 exhibited here, Birders are the Lenses main stream users, followed by people like myself, Wild Life at a distance.
The 200-400f/4 is clearly more your closer in Lens, with the option to go out to 560 albeit at a small light disadvantage with f/5.6, More suited to your Wildlife/Sports Photographer.
If I'm heading to Open Plains style Geography, Serengeti, Mara etc, I would place the 600 +1.4x in the Bag first, the 200-400 second.
If I'm heading to Okavango Delta, South Africa, Timbavati, it's the 200-400 in the bag first with the 300f/2.8 second.
In my own Imaging if I had to choose just one Lens for 90% of my Imaging, it would be the 200-400.
Thanks eml58 for a clear drawing between the two
Be able to visit places you mentioned above would be a dream come true for me - maybe one day when the kids grow up a little bit more.
I live in California, 10min -15mins away from Huntington Beach. It's very well known place for surfing. The summer here is quite special. Be able to capture the pro surfers will be interesting in my photography collection.
Great birding on that pier too. not often you get to shoot above a bunch of large birds while they go fishing. I was down there for a wedding recently (which means I now have relatives in the area) and absolutely wish I would have brought my 400f5.6.