It is truly two different ways of doing the exact same thing. Watts is simply the amount of power provided after the resistance of your atomizer, clearo, etc. If you use the voltage feature on your
device, you will still be producing watts (power), the watts feature simply allows your
device to change its output voltage to maintain the same amount of watts (power) regardless of what atty, clearo, etc you are using, to a point at least.
Either feature will provide you with the exact same vape, it's just a matter of preference honestly.
Maybe this will help a bit.
Amps=Voltage/Resistance and Watts=VoltagexAmps