100-400/4.5 vs. 200-600/5.6

[wwwprof]

I am currently shooting wildlife (primarily birds) with a 100-400/4.5 on a 6500. I was considering trading the 100-400 in on a 200-600/5.6 for the extra reach. While most of the reviews I have read state that 200-600 is preferred (especially if you don't have either, unlike myself who is looking to "upgrade") I have also read several reviews stating the 200-600 needs really good light or the results disappoint due to high ISO. Also, I am concerned that trying to acquire initial focus at 200mm vs. 100mm might be frustrating. Anyone with experience with both care to share an opinion? Thanks in advance.

 

[Ziggy]

The bigger zoom is very much larger and heavier, so if you're a mobile shooter that'll cramp your style somewhat.
An alternative is to use a 1.4xTC to get more reach. The 100-400 copes well with it IME. Downside is f8 which makes subject isolation hard esp. on APS-C.