Dude, buy a good f'n bulb.
you can buy a couple of the 150's if you want to, but if you want to be serious, get the 600w
150's don't have the same quality of the 600w bulbs, and 600's typically provide more lumens per watt than ANY wattage hps produced.
only reason to go with multiple 150's over a 600 is if you're doing a sog setup, because you can use one light for each stage of plants.