A mini battery like the Bloog will give you 1 -2 hours on a charge. An eGo type will give you 5 - 6 hours on a charge. And the other thing to consider is that as a battery drains, the performance suffers even though it still functions. So the higher the mAh rating of the battery, the longer you will have a decent vape. And you can use an adapter ($3) with any model so if you prefer 808 cartos/attys, you can still use them.
However, I would not get the eGo because it is only 3.4 volts. A better option in the same physical size/category would be the E-Power 510, Kgo or Riva which are true 3.7 volt models and come
standard with 1000+ mAh batteries at no extra cost unlike the ego. The E-Power now also comes in a 5 volt version which will give you much warmer vapor:
Electronic Cigarette - Starter Kits - EGO E-Power
Plus, manual batteries provide better control, better throat hit and are safer.