If you're leaning toward a REO, get the VV. Battery drop-off is a PITA if you favor a certain heat. "The Kick" is a variable wattage drop-in which requires slight scuffing of the anodized surface inside the REO to get a good negative contact. It's finicky, tho. The Kick can be used in a regular REO Grand or the VMod XL. While the REO is more durable and designed/built in the USA, the VMod XL is not a bad bottomfeeder. It requires a tip to feed system, so there's less chance of leaking. It has a slightly different feed system. It can also accomodate a Kick with some slight modification. My opinion is that the VMod XL is more comfortable to hold, is more pocket friendly and less likely to have the connector threads damaged by lateral force in a pocket. They both have merits. The REO has a more vocal following. I have nothing bad to say about the REO.
Variable Wattage mods and circuits read the resistance of your atty/carto and draws amps from a high-drain battery in order to boost voltage to attain the heat or watts that you have it set at. Basically VV without having to understand how ohms relate to watts and voltage.
With a Kick you sacrifice battery run-time as it requires a reduction in battery size from 18650 to 18500. As it draws amps to create the voltage for the watts it eats the smaller capacity 18500s in about 5-8hrs on average by my account.