Not an easy decision, and definitely not a cheap one. I had the old 500 f4L IS MkI, which was due for retirement. I initially decided to go for a 400 f2.8L IS II, because I could get 640 and 800 with extenders and both respectable IQ and autofocus. Then Canon released the firmware for f8.0 on the 1DX and the 600mm f4L IS II temptation became a bit too much. However, just having the centre point for AF is a bit limiting, so my experience is that I seldom went beyond the 840mm with the 1.4xIII extender.
I then upgraded to the 1DX-II, which provides (near) full AF functionality at f8.0 and expected to use 1.200mm quite a bit. However, my experience so far is that I keep on using 840mm as the max. 1200mm is OK for static subjects, but a bit long when they start moving around. Another thing worth considering is the effect of humidity and haze. At 1200mm you will often have a lot of air between you and the subject and the images will suffer.
A 1DX/7DII combo, with the 500 you have is very good. The 600, on top of being quite expensive, is larger, heavier and that extra 100mm is not That much. I would rather spend my money on a 1DX-II and/or a 5DSR.
I am crossing my fingers for a 600 f4 DO, or even better, a 1000mm f5.6 DO.