400 or 600 hps what do u think

[clown baby]

If you're buying new gear, my vote would be for two of those 315w CMH/LECs.
Might cost a little more up front, but the bulbs last longer and make it back in reduced electricity usage.

 

[Can of Bliss]

2 400s would probably give you the best gram per watt. 2 600w would be overkill, but that means better performance in the corners and a better yield for the space. A lot comes down to how you want to grow. I'm all about the vertical grow, which is a different animal altogether.
A wattage switching ballast that can run various bulbs is much better than dimming a higher watt bulb. Bulbs are designed to run at full wattage. As a hortalux rep said
"if you want to see how the spectrum of a five year old bulb effects plants, put a new bulb in and dim it down to 75%" (not exact quote)

 

[Megaprop]

2 x 600s would max out your space if you can handle the heat, but 2 400s would probably give you a better grams/watt figure in that size space.

 

[Muleskinner]

So the grow area is 3X6? 400w is perfect for 3X3, 600w good for 4X4. So you can have one 600w HPS with poor coverage along the sides, or two 400 watters.

Personally I don't believe in running lights & ballast on dimmed settings. It fucks up the spectrum of the bulb and reduces the life of the bulb, both of which reduce yield. Leave the dimming to LED lights IMO.