If you are asking which card will give the highest burst rate, or give you fast downloads, you may look at the test figures in Rob Galbraith's CF/SD/XQD database here: http://www.robgalbraith.com/multi_pagee519.html?cid=6007
Since the 6D hasn't been specifically tested yet with these cards, I suggest you look at the 5D Mark III and 7D series as well as any card reader you use with your computer (if you do use a separate reader or are considering purchasing one).
Many pros I know use Lexar Professional or SanDisk Extreme Pro cards for their combination of high burst rates, low failure rates, and durability. They also come in high capacities, something you'll need if you shoot much on a given day. But if these are out of your price range, there are lots of other choices. Generally you get what you pay for, at least from what I've seen.
To estimate capacity required, you can guess that typical file size might be around 25MB with your 6D (the size varies considerably depending on settings and subject) and multiply that by your largest number of shots in a day with your previous digital camera. And if you're a good engineer, you'll pad that figure by ten percent.
[When I stepped up my maximum shots per day doubled, so take that with a large grain of salt!]
And yes, SD cards are typically slower than CF. Sorry 'bout that. As Rob's tests show the manufacturers' ratings mean little when compared to real world performance. In my view SD card slots are a drawback of many current cameras.
Hope that helps a little.