Another factor, and this applies mostly to MH
bulbs, is the depth penetration. 1-400w bulb will penetrate deeper than 4-100w bulbs. A MH
bulb is good for about 2' of tank...ie...for a 4' tank, you'd need two of them.
If you're using, for example, PC
's...then you wouldn't really see a big difference between 2x65w bulbs and 4x32w bulbs assuming they are oriented the same way. It's best to go with the bulbs that fit your tank with fluorescents though. For example, a 96w PC
is about 34" long...perfect for a 3' tank...two of them will span a 6' tank just right.