First question is, do you absoutely need something the size of a cigarette? Or, would you be o.k. with something the size of a Sharpie marker? That's going to make a big difference in your battery life, and can also make a big difference in the quailty of your vapor. It's especially important for a 2+ pack smoker.
Small batteries have limitations that no manufacturer can get around. It's physics. The best cig-style e-cig out there, the Volt from Smokelessimage.com will have a battery that lasts maybe 3-4 hours. A larger unit will give you 10-12 hours or more.
Batteries are rated by capacity and voltage. Capacity is designated by mah. A 1000mah battery will last 4 times longer than a 250mah battery. As a rule of thumb, you can expect about 1 hour per 100mah.
There are two models of e-cigs that are recommended most often to newish vapers by the veterans on this site. They have been time tested and have had the most successful track record. Everyone has their favorites, and you will be inundated by opinions from people who are all bubbling over with enthusiasm about their first "real" e-cig, or the lastest advanced model they just paid $200 for and are convinced is the be all and end all of vaping.
But if you want maximum vapor, refillable cartomizers, and a no-fuss economical e-cig, two models stand out. One is the Volt. I only recommend this one if you absolutely must have something cigarette-sized. It is the best of a poor performing class of e-cigs. You can also buy blank cartridges (cartomizers) so the issue of nicotine strength and/or flavor and vapor production can be addressed by what juice you use.
The other model, superior to any cig-style e-cig is the kGo. It is based on another lower powered model called the eGo, but it comes with no fincky "systems" for juice delivery to the heating element. It wasn't designed to make you buy cartridges or atomizers or juice tanks from the manufacturer. It's compatible with virtually all types of juice delivery systems and you will easily be able to find the attachment that gives you the vapor production you're looking for. And it's inexpensive and dependable. The battery is 1100mah and will last you about 10 hours between charges. Kits come with 2 batteries so you can use one while the other is charging.
This site has a variety of kGo kits. All come with the basics of 2 batteries and a charger. The differences are in the included accessories. I would recommend you get the atomizer kit and add 2 boxes of 5 cartomizers. Then, you can get juice from anywhere to fill your cartomizers. With that, you will have a couple months, at least, worth of supplies to get you started in a serious way. Here is a link to one vendor. There are others with similar kGo kits.
KGO