VV you need to know your resistance so you can fine tune your vape to the wattage you like. A lot of times doing this, you basically just adjust to your taste. With VW you set your wattage, and regardless of your resistance, it hits your sweet spot automatically. It reads the ohm of your topper, and adjusts the power to hit the wattage you have set; without you needing to fine tune it.
Example. I like to vape around 10 watts. With a variable voltage device, using a 2.0 ohm carto, I need to set my voltage to 4.5. If I replace the 2 ohm carto with say a 3 ohm, i need to increase the voltage to 5.5. With a variable wattage device, i just key in 10 watts, and I am done. No matter what ohm topper I use, the device will hit my 10 watt mark automatically.
Hope that helps. Once you get a feel for it, I think you will like vw more. It's just easier.