eGo, like Riva, is essentially a big battery with a 510 connector. The higher capacity battery is important if you want to run lower resistance (for more warmth/power/flavor), as well as for basic longer run time between recharges. Both Riva and eGo have, I believe, both standalone batteries (ones with a separate charger) as well as sit-and-go types (ones with the charger built into the unit, you connect a microUSB cable to the bottom of it to charge).
Newer Rivas run at a bit higher voltage nowadays, somewhere around 3.7V fresh off the charger. I believe both Riva and eGo now also offer variable voltage variants (I'm certain about eGo, which has some units allowing several discrete choices, not so sure on Riva.)
(Personally, I stay with the notcigs variable voltage mods, they just work well for me, but I try to watch what is happening with other PVs.)