I would go with two 400w lights. (actually, that's exactly what I've got) Perfect for 12 - 3 gallon pots, or 8 - 5 gallon.

With two lights, you have more flexibility than a single source of light. If it gets too hot on summer days, can stagger the timers. (first one on for 6 hours, second one on the following 6 hours)
In case of mechanical or bulb failure, you still have one working light so photoperiod doesn't get messed-up waiting for the UPS guy to deliver your replacement bulb.
If 1/2 the plants are sativa, and the other half indica, you have the flexibility to handle the difference in height without having to stack phone books and cinder blocks. (if the lights are on a chain or otherwise adjustable)

Having at least two points of light, rocks. :thumbsup: