Your asking a different q now......I'm pretty sure I read something about not interchanging different sized ballasts/bulbs but I'm not 100% positive.

Stealth if you're going to start a fair amount of plants you most likely WOULD want the 400 vs 250.....when you think about it a 2' x 2' area really isn't very big.....while it works for us remember we've only got 4 plants going under it at any one time maximum.