I have a 7D and a 6D. You're right, the 7D uses CF cards and the 6D uses SD cards. For each camera, I have a 128GB card and I've never been able to fill either up at any event I've video'd or photo'd.
Make sure you get a fast card, whatever you choose. There are different write-speed ratings for both CF and SD cards.
I bought this mint 6D the other day, my first FF camera. I've used the 70D for years, love it. I'm having trouble getting the 6D to display four or (ten?) images at one time on the display. I've followed the book's instructions and I get to the point where after hitting the search button twice I see "10 images" dialogue. I hit SET but nothing else happens. What am I missing? I've tried a lot of things. The camera seems to function completely. This has been my only glitch so far. Thanks!
Win 10 Home Pro
24 g RAM