Hold on pouring the floor, and install PEX tubing before you pour. Simple to install, cheap to heat, and its great. The floor is always warm, and once you heat it up, it is a huge thermal mass, so it doesnt take much to keep it going.
First, lay down gravel so that the ground is as level as possible.
Lay down 2" Styrofoam insulation.
Put down reinforcement wire.
Attach the PEX tubing to the wire using plastic cable ties. (There is a certain pattern for how you lay it down)
Attach the PEX tubing to a manifold and pressure test.
Pour the floor.