For a small space like that I would go T5HO or CFL's unless you can bring in fresh air and vent all the Heat HPS will bring. The T5's WILL get warm too, but not near as much and you can run them right over the plant tops.
You can get 3 foot T5 fixtures, (fill the sky is my motto) plus you can add CFL sidelighting to supplement the lower branches. Light penetration is just less with the T5's than with MH or HPS.
Shoot...it's amazing what I have seen people do with just CFL's!