Regular eGo outputs a Pulse Width Modulated voltage equivalent to about 3.4 volts.
A VV eGo has 3 ranges, the middle about the same as a regular eGo, the high range about
equivalent to the Riva, straight through battery voltage, about 3.7 volts under load with a fresh charge, slowly decreasing to cut off.
The LavaTube is a true variable voltage
device adjustable to a somewhat higher voltage than either the eGo or Riva.
If you do not abuse them all are excellent
devices. The 'level' output of the regular eGo pleases some, the slightly higher output of the Riva (but fall off as battery discharges) hits harder, and the LavaTube, or other Variable Voltage
device can be adjusted to 'fine tune' the hit.
All three types may have problems with very low resistance attys and cartos.
Hope that helps.
Rocket