The difference between nichrome and kanthal is that for the same gauge, nichrome (also known as ni80) has a much lower resistance per mm of wire. Because of this, between builds of equal size, a nichrome build will have lower resistance.
If you use a mech and have a fixed voltage to vape at, this translates into higher wattage.
You need to understand ohms law; the wattage you vape at doesn't really have much to do with coil material. The watts you can safely vape at depends on your battery, device, and atomizer.