Cartos for ease of use when you are out and about, dripping attys, geni-tanks, etc, for when you have a place to sit down, I reckon.
It will be personal preference, really, after having a go at a few different devices.
+1
Personally i find using a VV or VW (vari Voltage or Vari Wattage) is my staple device type, the are several reasons for this of which one is the ability to alter the taste and hit you may require at different times of the day or night or indeed for different juices on the fly, a big device (18650 etc) is not always a requirement for this, a "Spinner" or "Twist" is about the same size as the bigger mAh Riva / eGo types but gives you versatility as to the taste you need.