Type of photography
The need for speed is dependent on the kind of photography that you do. Let’s say you are into sports photography and you use a camera that can shoot at 7 frames per second in full resolution with AE/AF. For capturing and writing the images, when you are shooting in a continuous burst mode, it will require a memory card that can handle that much amount of data quickly. A slower card with an associated slower write speed will severely impact the performance of the camera. For users with fast cameras, a Class 10 or even a UHS-1 memory card is better suited, i.e., unless your camera supports UDMA CF cards.
If your camera supports UDMA CF cards then you will be better off using one to use its full potential. A number of professional DSLRs comes with both CF and SD card slots. If you are shooting videos, a Class 10 card will give better write rate for compact entry level and even semi-professional DSLRs. On the other hand if you don’t shoot fast action or RAW or even HD videos and only need to shoot stills in JPEG format, then a Class 6 card is good enough, but we recommend a class 10 for best results in all areas.