What speed is the card that you use in Canon 50D? Do you shoot RAW images? This is only a personal point of view is that you get the fastest card possible depending on specifications of your camera and budget. If you get a slow card then you will have to wait for the camera to write to the card. This can be annoying when wanting to take individual shots quickly. If the card is capable of being written to quicker than the camera can write you should in theory not have a hold up while waiting to take the next shot. Hope this helps.