Even with a variable voltage unit, ending work, the watts, is still the same, bump up voltage to increase the wattage output. Standard Ego @3.7v with a 1.5ohm coil, using the formula V x V / R would give you 3.7 x 3.7 / 1.5 = 9.13 watts. An Ego Twist which ranges from 3.2 to 4.6 or 4.8 volts output, adjusting that dial at the bottom, increased voltage output to dial in what flavor and vapor you are wanting, in the end increased the wattage, example, set the ego twist to 4.2volts, we get this 4.2 x 4.2 / 1.5 = 11.76 watts. With variable wattage taking the major role in mod design, the micro processor in the mod does all this math for you, set to 10 watts, the mod read the resistance of your coil, and automatically adjusts the voltage for you, making it plug and play.
Doesn't matter the resistance of the coil, if the mod can give you say 15watts on a 3ohm coil up to its maximum voltage output it will just as it can do on a 1.5ohm coil. A lot of these newer VW mods do adjustments in 0.1 watt increments which really allows super fine tuning on vapor output, where variable voltage gets in the general ball park, VW allows hitting the sweet spot dead on.
A good analogy I generally use -
VV only mod = Stock Chevy Compact, Resistance = the road you are driving, take a trip from Southern Los Angeles, heading up to Bakersfield, as you climb hills resistance increases, going down hill resistance decreases, so going up hill, you yourself have to step on the gas more to maintain speed (wattage), go down hill you have to let off the gas so you do not go over the speed limit.
VW Mod = Same Model Chevy Compact with the upgraded luxury package with cruise control, set the speed limit you want into the cruise control (wattage), take the same trip, the computer adds more or decreases the throttle as you go up and down hills on the road
In regards to should you get a 50watt mod over a 30watt mod is all dependent upon you. 1) Larger more powerful mod say iStick50w over an iStick30w, the iStick50w will come with a 4400mah battery pack or so over the 30watts only 2600mah battery pack, more mah, longer time between charges, even if you are running only 25watts max all day, the 50watt will last longer than the 30watt on its charge. 2) The 50watt will give you more room to expand with in the future, say getting into drippers and lower ohms, that extra 20watts could be the difference between meh its good and works, to OMG vaping taste smexiness. 3) The 50watt will be rated higher in its amp limit of what it can handle over the 30watt, which is an extra margin of safety between a pocket fire just getting things toasty before you realize it, to that pocket fire literally being a fire and a trip to the ER.
hth