Best resistance on the head depends on where you like to vape. With a 3.0 ohm carto for example, you will only be able to push the wattage to about 7.7 (that may very well not be a problem, but bears mentioning). It will be easier on the battery...it's counterintuitive, but higher volts and higher resistance is more efficient and easier on battery life at any given wattage.
With a 2.0 ohm head, you will have a greater range with this particular device: up to about 11.5 watts, and you're not likely to want to go any higher, if even that high. However, at the same 7.7 watts on a 2.0 ohm cartomizer, you're going to be turning the device to about 3.9 volts and using a lot more battery juice (2.4 amp draw versus drawing 1.6 amps on the higher resistance head. Your batteries will last a lot less time at any given wattage.
Hopefully I didn't make that too confusing.