Riva/eGo/510 are a common family, all around 3.2 - 3.4 volts nominal, though I've heard at least some current Riva units output up to 3.7V at the start (it's been a while since I've used 510). That said, the liberty flights basic Riva kit at $30 after ECF discount is sweet. I like them with Boge 2 ohm cartos.
For "more" other than more battery life (larger mAh), jump into mods. I'm using a Notcigs Infinity, really like it a lot. Variable voltage, good run time between swapping out the batteries. You don't need LR carts/cartos, as they run between around 3.6 and 6 volts (adjustable with a thumbwheel), you use the voltage to get your desired power rather than extra amps. Notcigs is coming out with a Buzz Pro next month, with an Infinity Pro around late July or early August. You can get either a 510 or an 808 fitting. Price-wise, my Infinity with three pairs of batteries (you use a pair at a time within the PV), ran about $120. I believe the new ones will be a bit more expensive, though not much. I run 5 - 6 ml of liquid a day, very rarely have to swap out batteries during a day.
I actually started with 808 gear, standard batteries and then the larger G400. I liked them, changed to Riva for 3 months due to the combination of good price point and roughly equivalent vaping (with the Boge LRs).
Is it worth the $ (price of 4 Riva kits) to go to a mod? In my opinion yes, although standard Riva kits are available at a good price. I really like cartos (808 has better selection, generally), and up around 5V is my sweet spot, though I vary the voltage depending on particular carto or if I change liquid. I had been off cigs for some time, knew I was a confirmed vaper, before buying the Infinity, so was happy to spend the $ on something I was pretty sure I would get good use out of.
