With the igniter in in there and the pressure of the sodium bulb it tends to run 5-8 degrees (F) hotter than a metal halide which does not require "ignition".

Hope this helps.. I've always found HPS to run slightly warmer than MH .. but not by much.

Either way with a MH in there you should be ventilating anyway.. so the temp of a HPS should be as equally controllable.