Riva 510 KITS from Liberty-Flights.com and their loose Riva 510 SE batteries are definitely 3.7v. You would have to ask them about any other loose batteries (they could be older stock which is not 3.7v). 3.7v is about 4v fresh off the charger, 3.7v plateau, drops off near the end of charge. These will vape any particular carto a bit hotter than EGO does. LF had some new black SE kits with 3-click enable/disable (nice for pocketing)
Ego is around a more steady 3.4v. The Ego-T batteries have a 5-click enable/disable (handy for pocketing). But I think about 50% of people who try tanks don't like them (leaking, lack of flavors, your flavors are still mixed in an atomizer, trouble keeping the atomizers clean etc). I only recommend Joye (or Janty) brand for Ego - best way to avoid crappy clones.
Both use cartomizers equally well. Riva is a bit more powerful in voltage, Ego is a bit more steady in voltage. You can always use different ohms cartos to get the heat you want. I like to use standard resistance cartos because I move them between a slim 510 (can only use standard resistance) and the Riva (gives me more heat and vapor out of the carto).
Boge are the most popular cartos for 510 threaded devices. Some kind of Ego Mega Dual Coils are also very popular, but use your charge up twice as fast and can put too much strain on some batteries. Many use them, some damage their batteries.
