As expected, LED lights were by far the coolest to run.
What did surprise us, is that adding a 2nd LED didn't really add much heat. Air temperature was still 3 - 4oC lower using 2 x LED's than it was using a single 600W HPS.
Based on these results, you could go to 3 x 240W LEDs (720W total!) and your tent would still be cooler than when running a 600W HPS.
As a bonus, the humidity was also higher when using LED lights! This helps plants retain water as they don't transpire as much.
CDM lights weren't too much warmer than LED lights - also a great shout in summer.