400w or 600w for this space?

Hey everyone just a quick question that im hoping a few of you will have personal experience with.
I have a secret jardin dr90 3x3x6.5 i believe
Im about to upgrade my lighting, im trying to decide what will be better a 400w or 600w light.
It will be a digital ballast so i can veg at a lower setting and also to help with heat, which is my main fear!
We are approaching the warmer months where i am and its very common to see solid weeks above 35c
I'll get a cool hood and have a fan exhausting the heat as well as another fan with a cf (the cf wont be used until flower) for ventilation.
I have a portable a/c but no real air conditioning in the house.
Also i will be able exhausting outside through a window.
I plan on using a soil/perlite mix for medium if that helps.
But yeah would it be a bit tricky cooling a 600w in this space under these circumstances? will a 400w still put out too much heat? etc etc

I have the same tent with a 600w hps. You'll need some kind of air cooled reflector. I'm using a cooltube. I use passive intakes (3) and exhaust through the cooltube. The intake end of the cooltube has ducting attached to it and is the intake end of the exhaust.

Thanks chef!
What do your temps sit at? both inside the tent and also the room its in?
Also what cfm are your fans rated at?
I was hoping to get away with a fan for cooling the bulb, a fan for the CF and ventilation and then just an ordinary fan to keep air moving inside the tent.
On hot days i was thinking of setting up even an esky full of icy water outside the tent and have a fan blowing over top of it into one of the windows of the tent.
Has anyone else tried something like this?