600w HPS vs 1000w HPS

I am currently starting 6 green crack (hate the name, love the bud, haha.)
MY question is if i plan on topping or using the F.I.M. method to increase yield, would a 600w light be good enough or should i go ahead and get the 1000w to increase overall yield???

ALso i have my next setup in mind is to double the garden to 12. Should i get the 600 now and then another for the 12 plant setup?? or is 2 1000's recommened.
I have heard so many differnent sides on this.

the room is 5.5' X 4.5' X 8'

please somebody with experience help out...

thank you!!

From what I've heard 2 600's is better that's what I use, however I've never owned 1000 watt lights.