I have the 300 2.8 mk1 and the 600 f/4L mk1 and use them on a 5DIII. With a full frame camera you need to be able to get to 600 one way or another for wildlife. I've contemplated downsizing to the 500 due to the weight issues but would seriously miss the extra 100mm. On the 7D, I can use the 300 + 1.4x and get about the same.
I've even considered trading in both the 300 and 600 for the 300mkii if the IQ would stand up with the 1.4iii extender. I'm about to get one on eval so I can make that decision in a couple of months.
So my friend it's all about what you shoot the most. with the 500 F/4 you lose one stop over the 300 2.8, you lose some flexibility of being able to use extenders and still get a lot of light, and you probably will lose a little bit of AF speed. On my 600 the AF speed is ok with the 5DIII but I swear the 300 2.8 +1.4iii AF's faster and more accurately. The 300 combo also seems to better track eagles flying toward the camera (i.e. more keepers but more cropping required).
I too think the 200-400 would be great but waaaaayyyyyyy out of my price range. Spending 4k on the 300 2.8 was about my limit and getting the 500 is only feasible by trading the 300 and a wide angle plus putting up a few hundred cash on my end.
Congrats on the 200-400!!!! Especially on a crop body you basically have a 320-896mm zoom....covers pretty much everything