I think my 400 watt MH used to run cooler than my 400 watt HPS, or at least it sure seemed like it. That makes sense due to the fact that an HPS puts out quite a bit more light, therefore more heat.
That would be true if they were equally efficient, which they are not. MH is about a third less efficient at producing light than a HPS. If it isn't producing light, its producing heat.
Okay, yeah, that makes sense. I only used my 400 MH for about a week, before I ditched that piece of crap and switched back to fluoros.LOL Watt for watt, MH can't compete, IMO.(sorry to drift off topic.LOL)
^No doubt, especially if you aren't vegging some monster plants. I like using them for plants that are no bigger than 18", because then light penetration starts to become a problem and they stretch a little.