Light switch with power to the light first
Depends on the location and age of the building. Modern U.S. code usually requires the presence of a neutral wire in all switch boxes. 

Look at the number of conductors entering the switch box. Two conductors, both connected to switch terminals, means older style wiring, and you are out of luck. You'll have to run the new circuit from the light. 

Three conductors, one capped off with a wire nut, means newer style wiring, and you can draw power from the switch box. The capped white wire is the neutral, and one of the connected wires -- usually the black one -- is the hot supply. But check everything with your favorite diagnostic device to make sure. 

As always, be careful not to touch any wires when diagnosing a live circuit, turn off -- and verify that it is off -- the power at the breaker box before handling the wires, and never change or install anything until you are certain that you know the path and purpose of every existing wire.