If you can set watts, do so. The device will check the atomizer resistance and set the voltage appropriately to make the power (watts) you set.
If you set volts, atomizer resistance combined with that voltage will determine the power you're vaping at (according to Ohm's law). If you change the atty to one of different resistance the power will change.
If you want to get an idea of what the power will be, here's a
online Ohm's law calculator. Plug in the ohms and the voltage and it will give you the watts.
The correct watts or volts you want to set your device at depends on the juice delivery device on your battery and the juice. Start a bit low, vape it and if you feel like it increase the power a bit. Vape it and repeat. If you get to the point that you taste a slightly burnt hit, back down the power a bit and that's the maximum you can get out of this setup. Chose a setting at or below that to suit your taste.