I prefer 4.0 ohm vision heads. Maybe because I use AW ICR 123 and Ultrafire WF-139 charger. The loaded batteries have 4,2 volts. When put stacked in GLV-2 (resisted version) they give approx. 5.6 volts at the output.
2,8 ohm heads can handle it, but one has to take care that the coil is wet with liquid (ie. don't take one draw after another, often turn the e-cigarette down under to let the liquid flow down to the coil), but, IMO, 2,4 ohms at 5,6 volt (or more) is a real nightmare.