I asked the same question when putting up my shit. I ended up going with the 1k and a large hood. I'm able to cool it and keep it fairly close to the canopy so I'm happy.
i have both [600&1000] a 1000w covers a 4x4 area good & im currently running the 600 & a 3x3 tub ebb & flow growing 9 plants but next grow im gonna try 6 plants here is a few pics of my current 9 plant 600w. hps grow the 1000w. will require more cooling but in your space 12 plants would work with 1000w.