Go and rent them for the weekend and decide based on your personal preference. Obviously there is a difference, otherwise people would not spend the money to get the 5D. However, whether this difference is worth the money to YOU can only be decided by you.
Lol! This thought came to mind, but forgot to include it. You can rent a camera for a few days for not a lot of money. I did that with an infrared 7D a year ago to see if it was worth converting one of my older cameras to IR. Working with the IR 7D was great fun, but I decided that infrared isn't where I want to be. I saved myself a lot of money on a camera conversion and still have a camera I can use as built.
Yes, rent both. Download the user manuals for each of them so you have a better chance of using them well while you have them. Shoot with them the way you want to shoot. Does the 6DMkII do what you need, or is it wanting? Is the 5DMkIV exactly what you need, or is it the proverbial "overkill"?