From what I have heard and read the Megaray is definitely the hottest of the mvb's.
I have never used the T-Rex, but have used the Mega and the Solar Glo. I actually did find out myself...that the 100w Mega puts out more heat than the 160w Solar Glo... if that tells you anything. The Megaray no doubt puts off alot of heat.
Hope this helps some