Personally I've been using Sandisk pretty much exclusively over the last several years and have not had any issues.
Currently I'm using Sandisk Extreme Pro CF cards now that the price has dropped a decent amount in the last year. These are HQ fast cards and should more than enough for a 6D. I prefer multiple smaller capacity cards (such as 16 GB) as opposed to single larger ones.
For your 6D you will want the SD since there is no CF slot. I buy mine through Amazon
If you want to spend a little less and are not concerned about the writing speed go with these
I've read good things about Transend but don't have any pesonal experience.
