In my experience it will cost you more to get a proper t5 fixture than an = MH fixture... BUT
at that point you start figuring in other factors like near endless color combinations, the heat transfer to your tank and so forth.
I've owned 400w DE MH.. 250wDE MH... 250wDE MH with T5 supplimentation.. and strait up t5.
Out of all of them I like the T5. I like the color... the heat advantage (yes I know the argument but it DOESN'T heat your tank like MH), the color options, and the electric bill.
One day i MAY go back to MH, but that day isn't today... or next week either.
B