Hmmm didnt know that about the 750s...I guess the 600 would be a good choice....I know the 600 is the most efficient in the traditional ballast world. I already have a 400watt HPS and a 400 watt MH that work fine...but knowing what I could have just eats at me. I may just rip the ballast out of my 400 HPS and use the hood socket and everything.