Variable voltage (VV) and variable wattage (VW) allows you to set the temperature of your vape. If it is too cool you wont get vapor. If it is too hot your juice will taste burnt. Anything in between is a matter of personal preference.
Variable voltage is dumb. It doesnt pay attention to the oHms of the juice delivery device you are using. You need to start low, vape and turn up the voltage to get the vape you prefer. If you switch juice delivery devices to something with different oHms you will need to adjust the voltage settings.
VW is smart in that it recognizes the ohms of the juice delivery device. For example, if you set 8 watts for a 1.8 ohm juice delivery device your PV will deliver the same temperature vape if you switch to a 2.4, 2.8 or 3.0 oHm juice delivery device.
I would suggest that you get a VAMO that has VV and VW. It is cheap, reliable and has enough power for nearly all but a very few people who prefer super high power vaping. Use the VW setting to start with and experiment with VV if you want to learn how to match oHms with voltage to get the vape you like.
You have to wait for shipping from Hong Kong but Fast Tech has VAMO kits with an excellent charger and batteries for a very good price,
Vamo-Vivi Stainless Steel Voltage Adjustable E-Cigarette Battery Compartment Set with Nitecore I2 US battery charger/charger cable/2*Panasonic CGR18650CH 18650 batteries.
I would highly recommend that before you spend much money on juice delivery devices that you try dripping. Dripping isnt at all convenient but it is the gold standard by which to judge other juice delivery devices. I have a large pile of juice delivery devices, including RDAs and RBAs. For my preferences, they all fall considerably shy of the quality of taste and performance that dripping delivers.
Ive found the HH.357 atty delivers best taste/performance of the atties Ive tried. I also have found it to be the easiest to keep correctly saturated. They are expensive but Hanna will rehab them for a $5 shipping fee.
Avid got a bad shipment of materials so the only HH.357s currently available are the customs. Get the standard resistance 2.5 oHm to start as that will allow you to explore the widest range of power settings,
Custom HH.357 Cisco Spec Hybrid 510 Atomizer.
You will have money left over and after using the VAMO/HH.357 combination as a tutorial you will have a good foundation on which to judge how to spend the balance. It seems Im a very picky vapor so I went for the dripping performance in a tank with Avids expensive AMP Tanks. However, tons of people are thrilled with their top or bottom coil clearo tanks, others are fans of carto tanks while some love their rebuildables. You are the only one that can decide what floats your vaping juice delivery boat.
You could also decide that a mechanical PV rings your chimes or a VV only device. You also might find that you are in the minority that wants VW but more power so a DNA 20 mod might be your cup of tea.