Just keep it simple ,find a PV you can easily identify and find replacements for ,the standard is 3.7 volts and the only difference is how long your battery lasts between charges.The stock 3.7 volt devices produce the same vapor from the same liquid on a fully charged battery. We all have our favorites and if you stick around a while you will no doubt see our favorites change to whatever the next one we buy is.Not sure why but no matter how well our rig is working we are on the prowl for something better.