I'm sure that UDMA6 monster would be great (especially if you want very high burst capabilities while shooting RAW stills). For video, 600x is actually a lot more than you need
I've been using the Transcend 400x 32GB (about 1/3 the price), and haven't ever had any issues while shooting video
The SD slot in the Mk3 isn't UHS, so even if you put a super fast SD card in there, it will work, but will only read/write at class 10 speed. My SD slot is usually left empty. Would've much preferred dual CF slots, but whatayagonnado