A common question and always fun to answer. The short answer is there really isn't any difference. You just set it where you like it and you're done.

Here's the technical answer. You have to look at what Volts and Watts are. Volts generate a current flow based on resistance and thereby an amount of power. Watts are the power the comes as a result of the resistance and the applied voltage. I'll spare you the math on that.

So, by regulating to Volts, you get a power level depending on the resistance you use. When regulating to Watts, you're accounting for the resistance so you'll get the same power level regardless of the applied resistance.

The main difference is that Watts accounts for the resistance and Volts requires you to manually account for the resistance. When the rubber hits the road, it's simply a matter of scale. You can achieve the same result with either Watts or Volts, but Volts requires you account for the applied resistance.