My 200-600mm is to arrive today, and I've had the 100-400mm for a while now. I am essentially considering them for two different purposes - the 200-600mm lens is for when I need pure reach, for distant, or very small birds in high treetops. Due to the close focus not being nearly as good, and the lens likely being a touch slower, I will switch to the 100-400mm when I'm shooting in situations where the reach isn't likely to be as needed, and when I will be shooting closer subjects and birds-in-flight. At least that's the plan. The 100-400mm is nearly macro level with its close focus performance at 400mm, and can even be paired with the 1.4x TC and not lose the minimum focus distance at all.
I've had the 1.4x TC for the reach when needed on the 100-400mm, but it's a bit too slow at F8 for dark forest situations requiring big hikes in ISO, and at very long distances, it loses a bit of IQ where I feel the 200-600mm lens will likely do a little better. Plus, I'd like to experiment with the 200-600mm lens plus the 1.4x TC, as that provides a ton of reach - though not sure if atmospherics and aperture will be impediments making it not worth pairing together.
I don't have any extension tubes to try on E-mount bodies unfortunately, otherwise I'd be happy to have experimented for you. I would probably consider the 100-400mm still the more versatile overall lens, with decent reach on the long end, ability to slap the 1.4x on for more reach, and much better coverage on the wide end along with excellent close focus ability - and from what I hear, still overall a faster focusing lens with the dual linear motors.