If you want to light the whole area (6' x 9') then youll need another light or a light mover. You could get by with 6' x 6' and rotating the plants often.

HPS are better for the flowering stage, like DJ said, but you can go the entire grow with one. You might see more space between the internodes which means the plants will be stretched slightly more then they would be under a MH during vegetative. If you found a really good deal for the 1000 watt HPS and are limited on funds then I would say go for it. Your best bet would be to get the HPS and buy a MH conversion bulb for it. Then you can use the conversion bulb for veg and the sodium for flowering.

Laterz