Sounds like you are using a tank system (remove small rubber cap, fill juice into plastic tank, pop cap on and punch onto the atomizer needle?). It's a cute little gadget, ... with insufficient fluid delivery. Take a step back and invest into an atomizer and a driptip (or cartridges), or take a half step back and get some cartomizers. The difference: Atomizer/driptip (or cartridges) need frequent refilling. A cartomizer can last you a few hours depending on the size you choose.
For a 510/510 Mega battery choose the SR's (standard resistance), else your juice will taste burnt after a few draws.
As far as the - I'm gonna call it - "bigger is better" goes: The eGo's are great and so are many mods (depending on the batteries that you use them with). The difference between the 510 and larger batteries is really simple: Longevity. A 510 will give you approx. 20 to 30 minutes of consistent draws without much of a change in flavor. After that the decrease in available power causes a decrease in vapor production and thereby taste. The longer the lifespan of a battery, the longer you'll enjoy a good tasting vape. That - of course - is affected by the atomizer, cartomizer or tank that you are using with the battery. Choose an atomizer or cartomizer resistance that compliments the battery.
Anything from eGo and up will support the use of LR (low resistance) atomizers and cartomizers. These will produce a much warmer vapor and more intensified flavor and that seems to be what you are looking for.
Good luck and happy vaping!
