I favor the faster CF and SD cards mostly because I can download them via my USB 3.0 card reader faster. Slower (Class 6) SD cards would cause problems with other Canon DSLRs (like the 550D/T2i and 600D/T3i). I stick with Sandisk Extreme (not Extreme Pro - though I have nothing against the "Pro"). Never had a problem with the camera writing to Sandisk Extreme's.
I'm looking forward to April's clean HDMI firmware update for the 5D3. I have a BlackMagic Design HyperDeck Shuttle (a very budget-friendly device, but with no monitor - only passing through to another monitor). It records from the 5D3 fine now, except that it isn't at full resolution and is only clean using Magic Lantern's Alpha release for the 5D3. There are some issues that remain to be seen with the firmware update. The biggest issue is how the HDMI output will treat the 24p framerate and whether it will have 24p out or if it will be embedded in the 1080i out. If you don't already have an external recorder, I'd wait to see reports of how each external recorder pairs with the 5D3 when the firmware is updated in April.
Thanks for the replies, are referring to Sandisk Extreme CF or SD cards in your post?
I was surprised to even hear about having to choose between cf and sd for performance, i will be using the mkiii 85% out of 100 for video, the rest for portraits, with the 6D, Nikon 600D and who knows what else is around the corner to wait till April sounds like bullocks, not very cool, obviously Canon is playing into the market and like Apple and other major players they probably have great stuff but holding back to see what everyone else is willing to put out for the price range, this is a drag because the only investment one can be sure of is glass here.
i am also looking into the BlackMagic Design "hyper deck", decent price, but again, new stuff around the corner means prices will drop.
Has the hyper deck given you any issues?, is it solid?
For the price of CF cards i cant see why anyone would not get this and rig it with a matte box and all.