SD cards don't fix in my XQD and CF Express slots, and they didn't fit in my CF slots. I don't know a single professional photographer that uses SD for anything but a backup and I personally use dual CF Express or dual XQD if I need a backup. But I guess everyone with a Canon rebel and a kit lens thinks they are a pro.
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Like for like, the CF Express is $37.75 more expensive for a significantly faster and more durable card. Why on earth would you cripple a camera and your workflow by using the backup slow as your main card?
I get if you just take one or two hundred shots a day that you don't need a fast card and you're ok waiting on the buffer clearing, but then you are also just fine using wireless transfer or plugging the thing directly into the machine.