Feed from the main bus and isolate both ends of the siding on at least one rail. That way it will be easier to cut in a switch (single pole single throw) that lets you turn off power to the siding. You might want to switch the power to the main too so that your trains have more options when passing each other.
Good Luck,
Karl
The mind is like a parachute. It works better when it's open. www.stremy.net
I would run the wires from the main bus. If it's a short siding, you might not even need extra feeders, because the power will be carried through from the main tracks via the turnout.
Be careful if you are using "power-routing" turnouts, like some Pecos. Some of these will have problems if you provide power to the frog-side rails and don't isolate the turnout.
It takes an iron man to play with a toy iron horse.