Okay, no fair- I was thinking "why not use BOTH? you could put the 3" in the unused space. That way, even if you don't have enough plants for 5 x5" and 4x3" ( 9 total) you could just tape/block the empties with wood".........
Then that damn Jedi got to it first. Fast at posting, he is.
But, Luke Highwalker, I think the limiting factor is going to be the light- have you considered adding some more? Some CFL's shouldn't be too expensive, and you can put them right into the plants, not the mother.
Let us know what happens.