what i learned was the more watts the better. it is wastefull if you have like 2000 watts for one plant but the more watts the faster the grow. im pushing 2 1000w Mh for my room which is 9' x 7' x 8' but dont have results yet. i heard that Mh should only be used for vegging and Hps should be used for flowering. if i was you i would invest on a ballast that can carry both frequencies so you can just switch the bulb when the time comes. as for 600 or 1000 i would go for a 1000w but if your grow room was 8 x 8 i would use the 2 600s but yeah, go for the 1000w