I like the 900mah eGo. They just work, deliver a better vape than the "3.3v" rating would suggest, are consistent (performance is the same whether they are freshly charged, or if I've been using it all day) and are not much bigger than the 650mah version.
A little technical question for you.. The batteries labelled 4.2v, do they just output the voltage of the internal battery, or are they regulated to give out a constant 4.2v? In other words, lithium batteries start out at 4.2v fully charged, and drop immediately once you start using them. It would be possible to output a regulated 4.2v but would need electronics to boost the voltage.