I also wonder if the writing speed of 30MB/sec vs 60MB/sec makes a difference. Do this writing speed make a difference when shooting 6+ frames/sec or shooting video?
Elmer
I have some different cards, including some Kingston 133x, Kingston 266x and Extreme IVs. I shot videos with all cards on my 5D Mark II, mostly the Kingston 133x, and did not run into issues. But when I am shooting hi-speed on my 40D or 5D Mark II I do notice a slight difference between the Cards in terms of continuous shooting, nothing huge but once in awhile I seem to be able to squeeze out a couple of more shots with higher speed cards....I have done a couple of tests where I would just point the cameras and shoot at a still subject, nothing very formal mind you, and just shot away, the faster cards would allow me to get a shot or two more before zeroing out. The Extreme IVs run 40MB to 45MBKingston Speeds, while the Kingston 133x are at 20MB and 266x at 40MB, click on the X-Speed tab here.
If you like I can load the cameras up and run some more formal tests....
If you look Speed Results you can see read/write tests for various cards.
Basically if I think I am going to need real high perfromance/speed I will load in one of the faster (more expensive) cards, but otherwise for general shooting one of the slower cards which still work well enough. The Kingston 133x and 266x have been reliable for me and no complaints or failures, though I would probably just buy the faster ones.