MH's give off a "white" light...better for vegging. I've seen enought pics to see the difference between MH and the yellow/red of HPS lights...

I would kill to have two 400 watt'ers....for flowering. But, first you got to get though this tricky veg cycle. You say, "old style" 400 watt lights?...magnetic ballasts?...if so...I think, they are better, more dependable that digital ballasts. CMH bulbs work great in magnetic ballasts.

...anyways...the Uv rays from MH's are tricky...watch for leaf burn. I found it's better to "offset' plants some, under the light. Seedlings are very, very tender. And rotate plants, too. (but, not with the light on, I burned my knuckles doing that...)