That's interesting - re Riva 510 and 3.7v - I was going by what seevral people have said on this forum as well as in the comments on Liberty-Flights. Originally the Rive SE at Liberty Flights was the only 3.7v one then someone said they asked LF and they said all of the current kit stock is now 3.7.
Interesting. Well at least it is definitely warmer than a slim 510. I move a standard res Boge carto back and forth between the 510 manual and the Riva 510 and the Riva gives me a bit bigger/warmer hit which Ego would also do I guess.
BTW - interesting old battery voltage measuring test (includes Ego). On this old computer I can't really read the multimeter display but it looks interesting
http://www.e-cigarette-forum.com/forum/joye-510/65055-battery-voltages-surprise.html
Re which you should get - you need to decide how important slim look is vs potentially warmer hits and long battery life. Or get both - some people do that to have something slim when in public and something more beefy at home and for long trips etc. I use both manual 510 and a Riva 510. I like the slim ones for most of my vaping, I have chargers at work and a nice roomy PCC from NoSmokeVA.com for the short 510 manuals. And I carry a charged Riva battery too (in a cloth-covered Chinese lipstick case with something wrapped around the middle of the battery so the button doesn't press on anything). Slim 510 cannot use the LR attys/cartos nor the fast charger of an Ego or Riva (any of those can damage slim batteries)