Volts and watts are two sides of the same coin, or (more precisely) two parts of the same equation, the third being resistance. But there's a big difference when it comes to e-cigs -- volts are set blindly, regardless of resistance, which means the same voltage will inevitably produce wildly different results with atomizers of different resistances. When you set your PV to a certain number of watts, on the other hand, the device does some calculations based on the resistance of the atomizer and sets the volts automatically. This is the magic, and it means that your coil heats up to the same temperature regardless of its resistance.
In short, volts are a dumb setting while watts are a smart setting. With volts, you have to do the calculations, with a chart you probably do not have handy, based on the resistance of your atty, which you may well not know. With watts, you don't have to know any of that -- the PV measures the resistance and calculates the right voltage. In other words, with watts you can just vape away without having to do extra work. That's pure win.
To draw an analogy with water, imagine that you have a kitchen faucet and a fire hydrant (that is, two different atomizers with two different resistances). What you want in both cases is 5 gallons a minute of water (or an atty coil heated to precisely the temperature that makes your juice rock for you). Using a specific voltage to get a specific vaping result is every bit as wrong as saying "I'll just turn the knob 1/4 of a turn" on both the fire hydrant and the faucet and expecting the same amount of water to come out. That will not happen, because the voltage / turns of the handle produce different results with different equipment.
Watts, on the other hand, let you say the equivalent of "I want precisely 5 gallons a minute", and you get exactly that. As I said above, pure win!
Watts are a no brainer.