31 inches is kind of small. I'd go with no more than 250 watts if u get the HPS. If not the spiral cfls work great for veg , minimal heating issues, just be careful with the humidity in the room . I have known somebody that shorted 1 lamp out before because the copper winding on the cfls popped. As for the T12s ,4 footers are excellent for vegging ! Get soft white bulbs if you cant find T5 fixtures. Also , you can go from seedling to buds with T12s or T5s, so if you're on a budget , go CFL your first grow. Good luck ,peace out !