well i had 300 watts of metal halide over my 75g tank and didn't have very much growth or "great" color. now i have 324 watts of t-5 and have great color and great growth. i went the t-5 route mainly bc i wanted to try something not many people had, also a lot less heat than metal halide. now if you were comparing 2 400w metal halides to a 6 bulb t 5 setup there would be no comparison. hth, good luck.
jake