Lighting Advice needed...moving to bigger grow

First of all, thanks to everyone on this forum for all your help in the past. I have had a couple of semi successful grow and ready to try again. Here are some of the details.

I have been using a small grow box (about 8sq ft) with about 10 23w CFL's. I have AC ventilation into the box so i have been able to keep the heat at bay for the most part.

my question is...I want to move to a bigger grow area, maybe 14-18 sq ft. and considering all my lighting options.

My thought is to go with 400 or 600 watt MH with cool tube and run the AC vent through the light. I an a little worried that in a small confined space that the heat might still be an issue. And not sure which is the right size. I am thinking 3-5 plants at the most at one time.

or I was considering going with T5-HO set up with maybe some vertical sidelights for better light penetration.

or I could always just beef up my CFL bulbs.

I am still fairly new and not sure which way to go.

thanks

Rocklobster4040