how many plants? you said 2'x2' so i would go with as about 10 t-5 bulbs. You can never have to much... Then again why not a 400wt metal halide. You can get 400wt conversion bulbs that are both metal halide and HPS and run off the same ballast.
just think 10 50 watt lights is 500 watts and five hundred watts of flouros is not even close to four hundred watts of HID.
A HPS bulb make 50,000 lumens for 400wt
a T12 puts out 2880 lumens for 40watts 40 watts x 10 = 400 watts or 28,800 lumens
Its up to you but i would get a Metal halide ballast that will run a 400 watt HPS bulb for flowering. Then all you gotta do is switch bulbs and change your timers