If you go with the Reeflux bulb...which is what I'd recommend....make sure you get the coral vue ballast too. This is one of the great places to order from.
http://www.reefspecialty.com/product_info.php?cPath=14_29&products_id=200">http://www.reefspecialty.com/product_info.php?cPath=14_29&products_id=200</a>
Now the diff between the 400 and 250....this is a good one. I personally would go with the 400. The 250 is strong, but with the 400, you can lift it off water much higher, get better spread and have little to no heat transfer.... and still have a HIGH PAR value hitting those corals :) If you were to go with 2 LBs however, I'd recommend going 2x250.