Whatever you decide do not buy Lexar they didn't work in my 5D Mk2 (and a lot of other Canon models apparently), I use Sandisk and have never had a problem.
Found this on the write speed test site listed earlier in the thread...
Note: It's a quirk of the 5D Mark II that after secure erasing, some older Lexar cards must first be formatted in the computer before they can then be formatted and used in the camera.Thanks, I had found that solution through the web when I had problems with an 8gb Lexar card I had bought, it would work once and then not the next time, sometimes the files would get written to the card and other times they didn't, I gave up and sold it on eBay. I sent an email to Lexar about it and never got a reply.
I did some digging around and found quite a few sites that mentioned problems between Lexar cards and Canon cameras covering all ages and models, so I haven't bothered since, it was just to save a few quid over the Sandisks which have never let me down before or since.
