Vapinjeb

Either... but I prefer VW so I can set the preferred Wattage, and the device will adjust the Voltage appropriately according to the resistance of my coil for a consistent vape.
Wattage and Voltage are interdependent on each other, based on Ohm's Law.  With the coil being a static variable, adjust either Wattage or Voltage, and the other must change to balance the equation (in simplest terms).
If you know Ohm's Law, you can set your voltage on a VV device to provide the desired Wattage.

Have ordered twice, both times were OK.
Remember not to be in a hurry. If you need it NOW, there are other sites more suited.

I have ordered from them. Had an Issue after delivery once, answered a few questions and problem solved. I will buy from them again.

Yes.

There are clones, average clones, good clones, better clones, and sometimes the clones are better than the authentic!  You have to judge the quality and price for yourself.  Some good cloning companies are HCigar and Infinite.  There are many other cloning companies, some are good, some are garbage.
You have to judge the quality and prices for yourself and determine which route you want to go (and how far your wallet will let you).

I am a Mech Mod only type of person.
Thinking of getting the copper Overdose, like the lines.
My Nemesis is my work horse; never fails, just does it's job.
As they say, when the end comes, only twinkies, cockroaches and Nemesis left...

That's completely subjective and you're likely to get several different responses.
Technically, I would say the "healthiest" wattage would be zero watts...as in not vaping at all (while also not smoking).
Different liquid flavors, different PG/VG blends, different nic levels, different coils...all are going to have some effect on the wattage. Personally, I usually vape between 3.8 and 4.2 volts on my regulated devices.  That is usually determined by the particular liquid I'm using.  Some I like a little cooler, some I like a little warmer. The wattage ends up varying a great deal for me based on which tank or RDA I'm using and the coils in those tanks or RDA's.  I use anything from .15 ohm coils in my RDA's to 1.2 ohm coils in some of my tanks.

If it will only fire when plugged into the USB, then one of two things is happening... 1) The board is shot (charging circuit), or  2) The battery is shot (incapable of holding a charge).  Personally, I would recycle it and buy something new, rather than chase-down the issue and try to repair it.  There are far too many good APV's out there for cheap prices!