Each "X" equals about 1.32 MB. I don't think there's any direct correlation between single and dual layer writing speeds, just dual layer disks tend to be a bit slower. As for a drive being "better" I'd look for actual reviews and such rather than simply drive speeds. Most all drives tend to work just as well as the other one. Your only real difference now of days is error correction (IE the ability to read "bad" disks). Some drives can read damanged disks better than others.