I've got both, Rick, and at the moment for wildlife in good light I'm using the new 100-400, and occasionally with the 1,4 mk3 extender (when of course there's a limit on your focusing points).....will keep both for a bit and see how things shape up, I'm very loath to part with the 2.8 lenses (I also have the mk2 300 2.8 and just got the new 400mm f4 DO mk2....which is a stunner by the way...!!) and have the same quandary there.
I suspect something will have to go, but it won't be the 100-400 or the 400 DO mk2 thats for sure!!
Enjoy the new lens!!
George.