The Epower and kGo are both quality bigger battery PVs that give a great vape and are very affordable. The eGo is also popular, but personally I thInk you'd be better off with an Epower or kGo. The eGo is only 3.3-3.4V, while the Epower/kGo are true 3.7V devices. 3.7V PVs provide alittle more "oomph" than an eGo and more importantly handle the lower resistance attys and cartos a lot better.
The Epower and kGo are very similar in size, price, and battery life. The main difference between the two is that the EPower uses non-proprietary batteries while the kGo uses proprietary batteries.
I have an Epower and love it. If you like the idea of mOds, but don't want something that's expensive with all the bells and whistles, then the Epower would be a good device to start with. Technically it is considered a tube mod, since it does use removable non-proprietary batteries like all the other mods, but at the same time it's really not like most "mods" bc it's an "eGo type" device just like the eGo and kGo.
The "eGo type" PVs are a great place to start for a newbie that's looking for their first bigger battery PV because their not overly complicated, their very easy to use, and they give a very nice vape with much better battery life than a slim ecig.
I chose the Epower bc I really like that it uses non-proprietary removable batteries. Epower batteries cost $4-5 compared to eGo/kGo batteries that cost $14-20, and the Epower batteries last longer before they need to be replaced. Epower batteries are 1050mah and on avg I get a day to a day and a half on one battery. I also chose the Epower bc it's much easier to carry a spare for it. The Epower batteries are small (about the size of a AA but alittle longer) so its easy to throw an extra in your pocket or bag and if needed unscrew the switch and pop in a fresh battery. With proprietary battery PVs like the eGo and kGo, to carry a spare battery you have no choice but to carry a second PV with you. The other thing that i really like about the Epower is that everything is replaceable on it so it's cheaper to maintain. With the eGo/kGo, it's one unit. So if the switch fails or you need to replace a battery, you have to replace the entire unit everytime. With the Epower though, everything is seperate and can be bought individually. So if a switch fails you can buy just a switch for $10, if a battery needs to be replaced you can buy just a battery for $5 and continue using your existing tube and switch.
That being said, the kGo is also a quality PV, even though it uses proprietary batteries. The batteries are 1100mah so they last a long time as well and most people can get a full day on one battery.
I think your def be happy with an Epower or a kGo, so I don't think you could really go wrong either way. They both give a great vape, and will have a LOT better battery life than any slim ecig that your using now