(Long time CR lurker/reader, first time poster)
I am in the process of making the same decision myself. I find the image quality to be equivalent in real-world shots, although when I set up my ISO 12233-type chart, the 100-400 fares ever so slightly better at 400mm. My concern is less about weight, and more about the shorter (retracted) length of the 100-400 compared to the combo. I intend to go on a few subsequent outings with the 70-200 II and 2xIII to see how the combo handles for routine use. If it's okay, I plan to sell the 100-400, with the proceeds going toward a 300/2.8 IS II (for times when my 600/4 IS II is too big to bring).
I had+have both, and got the 70-200 II plus TCs as a replacement for my 100-400, which I upgraded to a Sigma 120-300 OS as the main tele and made use of the same TCs. Sold the 100-400 (and 70-200 mk1) to cover some of the cost and obviously don't need to keep them anymore.
For the more travel situations I took the 70-200 II combo and I like using a shoulder bag (Crumpler 7MDH in this case). However the portability advantage of the 100-400mm soon became very apparent. As someone who is:
- more of a walking casual birder
- someone who prefers not walking around with a tele on the streets overseas; but still wanting "local bird shots" there
- just having a camera ready to go, e.g. in the car, or how I used to work next to an airport for the odd plane spotting
The constant assembly and dis-assembly of the setup was just annoying. To the point where I eventually picked up the 100-400 again when one came up for a decent price second-hand.
At the 400mm end (my copies anyway) the IQ is pretty much equivalent, slight edge to the 100-400. AF wise the 100-400mm "turns" slightly slower but is far more reliable for me. The 70-200 combo can suddenly just start hunting randomly in all but the best light. Weightwise, well, I've never been one to see the need for a tripod for a 70-200/100-400/50-500 style lens.
Obviously the 7-2 is a killer 70-200 and also a killer 280mm/4. But not exactly doing "wildlife" there typically lol.
TLDR - the portability is the winner for me ... and I do hope any new 100-400 II can maintain that.