The eGo is a regulated battery. While the battery is 3.7 volts, it is regulated to only deliver 3.2 volts. It will, however maintain the 3.2 volts until it is discharged. Some mods, such as ePower, will deliver regulated 3.7 volts throughout the life of the battery. Other batteries, like the Riva, are unregulated. The Riva battery will start out providing around 4 volts when freshly charged and it will drop off from there until the battery is discharged. I can always tell when my Riva is about to need re-charging because the performance will drop.
The advantages of the big batteries, IMO, is their capacity (will last longer) and their ability to provide more current under load (allow you to use LR atty/cartos).