Variable Voltage and Variable Wattage both achieve the same thing, increasing ending wattage at the coil, just to different methodologies. Resistance = A road and High way, voltage = gas/throttle, wattage = speed. Example and analogy is as such:
Stock Chevy Compact, you take a trip from your home to relatives in St. Louis, as you are heading down the highway at 75mph, to maintain speed going up hill you have to push down on the gas, as you go down hill you have to let off the gas, this would be variable voltage where you have to manually adjust the throttle yourself.
Same Model Chevy Compact with an upgrade package that has cruise control, you get up to cruising speed, set the cruise control to that speed, here the wattage like above is the speed limit, as the computer detects increased resistance going up hill or less resistance going down hill, it adjusts the throttle automatically to maintain speed for you.
Variable Wattage mods are a bit more accurate as well, but in simple principle, you set the wattage to your desired sweet spot, when you fire the coil, the chipset reads the resistance, sets the voltage for that desired wattage, then fires the coil.
OT
Both the Nautilus and the iStick are excellent step ups, and the iStick will allow future expansion upward for a little while if you progress.