BillB said:
The main thing is whether I will be able to take better pictures with the 6DII I than take with my 5DII. An important question for me is whether I will be able to see a difference in the quality of a 12x18 print made with the 6DII in comparison to one from the 5DII. That is one of my personal standards. If I can't see the difference in a 12x18 print, it doesn't mean much to me. There are some other things, but that is a big one.
I might be splitting hairs here but for the sake of accuracy here goes.
You, personally, will not "take better pictures" with a different camera, if, by "better", you mean more compelling.
Now will you be able to see the difference in 12X18 prints? That depends entirely on what you photograph, how good you are at maximizing the capabilities of your current gear, how you process your images, and how you print them. For most people most of the time the answer will be no, but don't think there isn't a huge IQ difference between the 5D MkII and 6D MkII, there is. All new Canon sensors contain much higher IQ than earlier generations, whether you will see the benefits of that depend entirely on if you see the breakdown in IQ of your current camera, if you do the 6D MkII will be a big upgrade, if you don't see that break down then the new camera won't do much for you.
As a roundup, if when processing you regularly raise the shadow slider, particularly if you also tend to raise exposure, you will see a difference even in small prints. If you ETTR and tend to lower exposure in post and/or leave the shadows slider where it is and lower blacks, then you will not see a difference in 8x12 prints between a 5D MkII andA 6D MkII.