two hps are better... just make sure to get quality horticulture bulbs, with blue spectrum... this will make all the difference... of course if i was you, i would go with one 1000, which will end up making about the same heat as two ballasted 400's

and of course if you could two hps, and 6 hours of extra mh in the middle of the 12, because more par'd lumens, is always key to a better harvest.

but you could be stepping on your dick if the heat isn't controlled