With a polished aluminum reflector the surface is going to be much smoother than white plastic thus you will get beter control of where the light is reflected. Plus I have to belive the metal will reflect more of the light than a plain white surface.
While this is true, polished aluminum reflectors are not cheap (like me). Flat white paint makes a dandy reflector, absorbing little light or heat. If you can't or don't want to afford the fancy ones, painting the inside of your hood flat white is a good second choice.