The big issue that you will see with a space that SHORT is heat. HPS is better than MH for flowering, but any HID will throw more heat than you will be able to deal with in the 2' of vertical space you have between bulb and dirt.
Look into T5 flouros (google 'new wave t5' for a good example) or a larger wattage flourescent bulb. They are out there, and they work. Best of all, they run cooler than HIDs.
If you DO decide to go with a HPS, be prepared to spend almost as much on ventilation/exhaust as you did on the light.