A larger battery ecig will give you more vaping between charges as well as the ability to supply more stable voltage to the atomizer. Vaping requires more frequent hits than a cigarette. Less nicotine is absorbed from the mist created by an ecig. With a cigarette, you are inhaling tar that stays in your lungs for a while. The particles of combustion are small enough to enter the air sacs in the lungs. Ecig vapor droplets are actually too large to enter the air sacs (a good thing). Most of the nicotine is absorbed through the tissues in your mouth and sinuses. Try exhaling through your nose slowly for a greater nic hit.
For $29.95 you can get a Riva 510 starter kit that contains two complete Riva 510 ecigs with 750mah batteries (5 to 6 hours heavy vaping between charges), a cone (dresses up the Ecig), and a charger, along with a nice leatherette box.
The Riva 510 is a 3.7v version of the Ego style ecig (3.4v). It will accept any accessory with a 510 thread. That opens up a multitude of options that you will see discussed here on ECF.
Click the 20% off icon at the top of the page to get the $29.95 price.
Electronic Cigarette - Starter Kits - Riva 510
It's also available in a Boge carto version for $34.99. You get all the parts in the Riva 510 Lite Bundle along with 10 Boge 510 cartos (a $16 value by themselves).