I only shoot fun videos of my kids, but i have some exp here. I use a sandisk ultra 30mb/s 64GB SD card. it's not fast, but it's roomy and it's fast enough for me. from what I've seen the 5dmk3's SD card slot is heavily handicapped, so i don't think any faster SD cards will show their potential. actually, my relatively slow card is beyond the 5dmk3's scope. On 2 occasions my camera stopped recording when iwas using this card and pushing it hard, ie 720 at 60fps all-i and 1080i at something all-i. I was just foolin around. now that i don't use all-i i haven't had a problem at all. as for flushing a load of RAW files, that's a different story, but not one any video shooter should worry about.