I wasn't "telling" anyone how to do anything. The OP was asking for opinions, based on the tent he/she has. I voiced my opinion...and my opinion remains the same (especially if the hood being used is not air-cooled or a cooltube).
I've grown many times with a 150w hps, and several times more recently with a 250w hps (in closets, not tents..with a S&P mixvent exhaust fan and 4" carbon scrubber). The end product was always very good, using either the 150w or 250w. My latest grow cab was my first attempt at using CFLs...and only because it wasn't feasible to use an hps in my current living situation. But, since I've used a 250w hps, and the OP was asking for advice about using either a 250w or 400w hps, I feel I'm qualified enough to offer my opinion here. He/she is certainly free to heed or ignore my advice. And, of course, you are free to voice your own opinion as well.