The following is my kind of summing up the pros and cons:
birds: 7DII unless in very low light conditions in which case 5DIII
action: 7DII or 5DIII depending whether you are focal length limited or not
astro: 6D maybe a little but how much less noisier is it?
portraits: 5DIII (the AF system for off center focusing in an eye beats 6D)
night and weddings\events photography: 6D,5DIII practically equal
I do have a 5DIII and after I sold my 5DII I was thinking about a 6D until I thought:
1. I will always try to decide which camera with which lens. Since you are thinking 2 different
type of cameras (1 FF, 1 APS-C) you will possibly do not have such dilemmas
2. It is nice for both cameras to be exactly the same in handling/buttons/menus/capabilities
But maybe apart from an addon in the joystick in 7DII I guess handling will be the same.
Still I didn't regret my 2nd 5DIII. I believe that no matter what you choose, you will enjoy your new camera(s).