I was debating between the 250w HPS and 400w HPS Grow System too. I'm also growing 6 plants and the size of my grow space (a closet) is very simiiar to yours. In the end, I chose the 400w HPS System because I wanted my plants to get enough light.

It's been almost a week with the new lights...and I totally agree with switchone. Having a proper exhaust system is vital. Even if its just a fan blowing the air out of the grow space. I'm cheating a little bit, cuz I've been using my 12,000 BTU A/C unit to cool my entire room. Withouut the exhaust fan or A/C unit, my temps would definitely be in the 90's. But with the A/C (10-12 degree impact) and the exhaust fan (2-5 degree impact), my temps are 74-80. Obviously, my electricity bill will be a bit more running a 1280w A/C unit...but I tried a proper exhuast system that ran out from my air cooled reflector and I couldn't get the same results when compared to my A/C unit.