I got started vaping with the Smokeless Image Volts. They are a high quality product. I did find it cumbersome to continually need to recharge the batteries, even though I started out with four. I only got about 3-4 hours on each battery, and if you don't keep up with recharging them, you'll end up with no batteries and no way to vape.
I moved up to the X2's. More vaping time with each battery. The 600 mah will give you roughly 6 hours, the 900 = 9 hours, the 1300 = 13 hours. Still small enough to pocket for on the go and able to use the larger juice carrying devices like the clearomizers, T-3, etc. You can also continue to vape while it is charging with the passthru connected to the USB-AC adaptor ($10).
I eventually wanted even more than these devices provided, and went with the big battery mods. If you get to that point, I recommend Smokeless Images non-affiliated sister company Altsmoke for the BB or Silver Bullet, or ProVape's Provari.