Another thing to consider is the amp limits of different devices. And the type of power delivery, Pulse Width Modulation vs flat DC output. DC signals are considered better because of the consistent power delivery. PWM quickly pulses power from 0 to 6+ volts and averages the length of the pulse to equal the setting.
If I remember correctly, here is a little list of popular devices and their stats
MVP2 = 5v or 11w, 3a, DC
Provari = 5.5v, 4a, DC
Vamo, SiD, Maxes = 6v or 15w, 5a, PWM
Kick 2 = 15w, 6a, DC
DNA30s (and likely the clones) = 30w, 10a, DC