You probably want the batteries to last all day, which means you want at least 1000mah. If you don't plan on vaping forever, definately get the 1300mah Spinner or Ego-C Twist (or something similar). They won't last forever and you can't replace the battery when it dies for good, but it won't matter if you quit completely before that happens.
If you want consistency and the same vapor production from fully charged until the battery dies, get one that's variable voltage like the ones I mentioned. You also want a spare battery just in case. You could get a 1000mah or 1300mah battery that's VV and a smaller 650mah as a backup. Then you have a small sized battery for when you're out in public or wherever. No matter what you choose, you're looking at less than $50 for two good batteries, and all the different juice devices are pretty cheap ($5-20). Unless you smoke a pack a week, you're immediately saving money by getting your e-cig gear!
If you want consistency and the same vapor production from fully charged until the battery dies, get one that's variable voltage like the ones I mentioned. You also want a spare battery just in case. You could get a 1000mah or 1300mah battery that's VV and a smaller 650mah as a backup. Then you have a small sized battery for when you're out in public or wherever. No matter what you choose, you're looking at less than $50 for two good batteries, and all the different juice devices are pretty cheap ($5-20). Unless you smoke a pack a week, you're immediately saving money by getting your e-cig gear!