SharpShooter Loc: NorCal

1826(is that your birthdate? ) welcome to the Hog!
Here's my take.
Of course if the rig is too unwieldy for you then I can see that maybe you can't use it.
BUT from a photographic standpoint, two things stand out. one that the f4 is a stop faster. That allows you to either double your ss or 1/2 the iso, both could buy you a lot of IQ, but for all I know you shoot only in very bright sunlight and neither is a factor.
Second, the 500 produces a MUCH higher quality Bokeh. Since those lenses tend to be shot wide open, there is always shallow DoF in an image thus the Bokeh comes into play producing much more pleasing images with the 500.
My experience is with the mkl zoom and the mkl 500. The IQ was very close but the 500 was a bit sharper(hard to see it) and it is longer. I tended to keep a 1.4x on the zoom but not on the 500. So that also always gave the 500 an edge.
But the 500 is virtually impossible to shoot BIF where the zoom was almost made for that, especially the mkl.
You might try the 100-400 with a 5Ds, you might be amazed by what a difference the 50mp can make!!! Good luck
SS

pbradin Loc: Florida

Do what you think is right for you and what you shoot and how you shoot. What the rest of us think is really immaterial. All of us seem to think, at least to some point, that others know more about what is right for the way we shoot than we do. If you have spent significant time with the new 100-400 and like the results, then that should be enough and you will probably be spending more time shooting because of the lack of weight on your shoulder. I am older (72) and ditching equipment weight has been a bit of a priority for me so I can do more of what I love without being exhausted.

jeep_daddy Loc: Prescott AZ

Yes, I'm sort of in the same boat. I started with the original 7D with the push pull 100-400. Then I got the 500mm lens and used it all the time. Then I got the new 7D Mark II and used the 500mm lens with that quite a bit and sold the old 100-400. So all I had was the 500mm and I also had a 300mm f/2.8 that I sometimes used. Then the new Mark II 100-400 lens came out but I didn't think much of it until a fellow named Jeff Cable, a pro photographer that does both Summer and Winter Olympics and takes people on African safari, said that the new 100-400 was so good that he leaves his 600mm f/4 at home now. He showed is Africa pics using the new 100-400 and I was amazed. By then the new Mark II was about a year on the market so I bought one. Now I hardly ever use my 500mm lens, but I've also slowed down on taking bird pics which I HAD a passion for. Not so much any more. I keep thinking that I'm going to get passionate about birding again but I'm beginning to wonder if I am or not.

gordone Loc: Red Deer AB Canada

I have both the 500F4 ii and the 100-400 ii and a 1.4x iii. I do mostly birding so the 500 with the 1.4x and 7Dmk ii is my go to lens. It is always handheld and I have gotten tack sharp keepers down to 1/25 s. I generally shoot at 1/100s or above if stationary birds and around 1/1600 for BIF. I use the 100-400 with 7D2 as my travel lens. If I take both lenses then I take my 1DX2 on one of the lenses and carry both of them. These lenses are all microfocus adjusted to each camera. The 500 with the 1.4x is still sharper than the 100-400 at 400 without the TC. These tests were done in an automated test system and graphed at each aperture setting so they are NOT subjective guesses. Both lenses are about as good as it gets for long reach with high quality images. I have never tried aftermarket lens and don't ever anticipate doing so when you can get this sort of quality with an OEM lens.