What is the best way to control speed on a steep downhill road Brakes or Lower gear?

It depends on the length and steepness of the gradient.

On a short slope, or one that is not very steep, use the brakes. But not continuously.

On a long or steep slope (as in going down a mountain), the brakes could overheat and stop working before you got to the bottom, or even burn up. In this case, it's best to use a lower gear and get the engine to do the braking, keeping your brakes cool in case you need them to stop.

In some extreme cases, you may find that even the lowest gears will not sufficiently slow your vehicle, due to weight, slope, or other factors. In extreme cases, a sustained high RPM in the engine can cause damage. You should therefore avoid routes which constantly tax your vehicle's limits in favor of less steep alternatives. In many areas, heavy vehicles are not allowed on the steepest sections of some highways.