I use both a ProVari and a GLV3 Mini, and like both of them. I primarily use the ProVari, but occasionally pull out the GLV3, just to change things up. The ProVari is variable voltage, so I can adapt it to almost any carto/atty combination. The GLV3 is fixed voltage (for loose values of fixed) and is between 4.2 and 3.5 volts, depending on the state of charge of the battery. If you're looking at just one device to keep you always satisfied, I'd recommend the ProVari. It's the most flexible.