Depends on how serious you want to get. IMHO the perfect size is 48x24x12. Have to go custom though. The 40g breeders are 36x18, so if you go MH you waste some light going with 2 and have some low light areas going with just one.
I have a 40g breeder as my frag tank and if it's sustainable, I'm going to have a custom one done. Can always do a 24x24x12 cube as well...