I'm into about of month of vaping, so I'm not a complete expert, but this is what I found...
It comes down to:
1. How long will the batt (battery) last until the next charge?
2. What kind of vapor production does the PV (Personal Vaporizer) give off?
3. How much of a TH (Throat Hit) does the PV provides?
4. How easy is it to lug around when you are out and about around town (size of the PV)?
For me, I am a chain vaper, so a batt that can last all day is better than the KR808's I started vaping with. I've learned that the bigger the batt, the longer it can go between charges. I've used a device that used a 18650 batt, and it was good all day with out a PCC (Personal Charging Case). VV (Variable Voltage) is great for controlling the amount of vapor production and TH. For me, the size of the device didn't matter, because I did not like lugging around a PCC, extra batts, cartomizers, and liquid. Because the larger PV can go all day without a recharge, I found that it was easier to carry just the one PV plus a bottle of fluid to top off if I was running low. The tank cartos (cartomizers) vary on how much liquid it can hold. I used a 6 mL tank and it had lasted me 2 days.
Also, I found that the less a PV looked like an analog, the less likely I was to get hassled by someone who thought I was smoking. And, ALWAYS say "Personal Vaporizer" to anyone in the public, because the word "cigarette" immediately gives off negative connotations for most people.
Once you answer those four questions, just go to any one of the vendors listed under "Forum Suppliers" to buy your mod.
Jen