Here is my $0.02 .... Go with the biggest lamp you think would work. IME the parts needed for a 70W retro will be about the same as a 250W lamp and maybe more than a 150 watter. I have a 250 W lamp over my tank and love it (14k phoenix, no supplements). I havent had any heat issues with it this summer, altough I do have a lot of fans and I use the AC.
What are you planning? Or are you setting up a new tank? just curious....
Brian