egos and rivas are basically a beefed up 510. Being a bigger battery (510 stock=180maH, original ego=650maH), they seem to have a heavier hit to them. I'm all about home-built 3.7v box mods now, though.
You can get monster ego & riva batteries in sizes like 750, 900, 1000mah... all good. One 900mah ego will last me all day, but the 510/ego only runs at 3.2v, and after 3.7 I really don't want to go back. My box mods run a 900mah battery & last me from rising in the morning till about 3 or 4 pm, at 3.7v. If you have basic soldering skills & like fiddling with stuff, check out the modder's forum. If it's more of a battery life thing, an ego or riva will do the trick.
A caution, though. Don't mix/match the chargers. Ego chargers push too many amps for a little 510; like trying to fill a dixie cup with a fire hose. An ego charger can make a 510 explode. A 510 charger *will* charge an ego, but it takes forever.
Edit: forgot to mention that DIY mods are VERY cheap to make. The one I'm using right now cost me about $12.50 USD in parts, and would have been less without the LED.