I've just got a Duracell 16Gb Class 10 200x SDHC card for the 70D that I'm getting very soon. I've had the same card in my 5D3 as backup and I've used it a few times when shooting raw & jpeg and never noticed any lag in the occasional burst shots I've done.
I had the same card in a 60D and I never noticed any lag in the buffer speed either. There's also the same card in the wifes 650D and she's used it for video and stills without any problems. You could spend a small fortune on the fastest read/write speeds available, but if you're not going to take full advantage of the speed increase there's not much point. For day to day video and stills the Duracell 200x cards are fast enough for me by a long way.