I'd say a 400 is plenty..
Let me put it this way...
I have kept a 2x4 tray with all my moms... and the clones on a table in the "wash" light... under a 400w t5..
I find it adequate...
it spreads the light over the whole tray evenly...
But t5s are not as easy to cool..
A 400 in a cooltube is like eating cake..
a t5 can't be cooled like that.. and believe me, they put out a CONSIDERABLE amount of heat...
I guess I would choose the 400 HID in a tight space... for the ease of cooling..
or a 400w t5 if it was in an open well ventilated space...
did that help? sounds confusing