Hi Vpman,
it isn't really about one or the other between pvs anymore

The mods out now have
both variable voltage and variable wattage. So you can use both on the same mod.
Variable voltage means you can change the voltage on your pv. When you do this you need to make sure the resistance (ohms) of your carto, atty, clearo etc is matched within a range so you don't under power your vape or over power and end up frying something by the amount of watts you go to.
Variable wattage has been around for quite a while now. The Darwin is a variable wattage mod. However they are virtually impossible to get. A lot of vapers added variable wattage to their pvs by using the Kick - a small piece of circuitry that can be put on top of a battery.
The latest variable wattage devices have variable wattage incorporated into the pv. You can set the watts (power) and the pv will automatically adjust the voltage for the resistance (ohms). You don't have to calculate using ohms law.
The newer variable wattage devices also have variable voltage - so you can use either on the same pv. Variable wattage is more convenient. You can change from a cartomizer to a clearomizer, or a tank while vaping, and stay at the same watts, or adjust the watts - and not have to calcluate whether your volts are ok for the ohms of your clearo, carto, or tank. The voltage is automatically adjusted for you. It's less hassle to find the right spot and stay on it. A lot of people prefer variable wattage now.
If you're interested in trying a mod with variable voltage and variable wattage then the Vamo is a good one to pick up - inexpensive and works well.