just to chime in, depends on whether you want to be seen with some big hulking thing or something more inconspicuous. tons of mods out there that are expensive and bulky, some are small too of course. less conspicuous thing are like the riva/egoT batteries/etc.
your power source has an impact on what rating of vaping devices you connect to it. some are only for higher voltages, some for lower. of course you can get a variable voltage device that lets you experiment, but after you find what you like, there's little reason for the variable voltage aspect i would think. granted, some will say some juices taste better at different voltages, but that tweaking isn't for everyone. i'll take a $5 bottle of any muscat grape wine to anyone's $100 red/white. but you may not...
cartomizers are an easy middle ground for convenience/longevity and most definitely price. something that holds 1.6ml of juice will last a bit, especially if you carry around 5 or so of different flavors with you.
i prefer dual coil 1.5ohm cartomizers, cheap as heck and each can last a couple weeks.
as for longevity, yeah, get something no less than 900mah of battery life.