Hey Sheer,
OK, the switch with the resistor was designed to use (2) RCR123a 3v
batteries. This is the standard switch that comes in the Prodigy. It regulates the batteries down to 5v, as Steve has found 5v to be the "sweet spot" as far as flavor and vapor are concerned for
vaping.
The unresistored switch allows users to either vape at 6v (using the same 3v batts as the other switch), or they can use (1) 18650 3.7v battery to vape at 3.7v.
The different switches simply offer different experiences to those who are interested. Personally, I find the 6v kills flavors and makes things taste burnt. 3.7v is much closer to what regular e-cigs run at. I hope this helps clear things up a bit!