400watt or 600 watt Ballast Please Help...

Heres the deal money is not a option just what is best. I am going to get a Lumatek or Galaxy, but my question is I am building a 18"x 36" x 72" grow box. You know one of the metal locking cabinets. See I might be moving and be able to build a bigger system so what I want to know is should I go with a 400 or a 600 ballast I will use a small section in the bottom so I will probaly flower with 60" of the cabinet. I don't want to burn my plants, but I don't want it too hot. Also I am using a cool tube reflector with a fully sealed duct and fan. Please respond soon I am ordering tonight. My cabinet is being used because it is the only thing in a decent price that is stealth. Throw your ideas on that. Thanks alot.
 
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