Educate yourself, my friend. You need to understand your options and what all of these new terms mean. That said, your question is not an easy one to answer.
Many people start with a cigarette-like form factor but find themselves dissatisfied by the short battery life, vapor production, and low e-
juice capacity. Only light social smokers tend to stay with these. (i.e. the eRoll or the Magma are two of the many examples)
Quite a few find the eGo size unit (i.e. the fat sharpie pen size) suits them. Some of the newer ones offer a measure of variable voltage to customize your
vaping with various e-juice. (Examples: the eGo, in many varieties/variants, or the Inferno)
A significant number of us have found the 'big cuban cigar' size (or pack of cigarettes size) 'mods' give is what we want and need. These tend to give the best vapor production, battery life, and e-juice capacity. These will most likely suit you best if you smoked more than a pack and a half of analogs a day. (eVic, Reo, Lavatube, Volt, etc, etc)
You generally will get what you pay for. Going 'cheap' can be an exercise in frustration. My best advice is to be honest with yourself and figure up how much you were spending on cigarettes per month and every month for the next three months count on buying ejuice and a new device each month. Try a good device from the lower category in the first month, then in the middle category the next month. If the middle device is too much, keep that for home use while your smaller carry-around-kit is charging. If middle is better, then in month three invest in a larger device and retire the others for emergency use or travel backups.
This only really covered the battery aspect of e-cigs. You need to work out the other two essential aspects of
vaping too. I.E. Which E-juice delivery system (cartos/clearos/tanks) and what e-juice (PG/VG ratio? nicotine strength? flavor?) suits your needs and desires.