I have a 400w MH as well with a conversion bulb for flowering and can tell you that the HPS conversion bulb works well - it was about $130 though, so if you can't afford it then the MH should work too. Unfortunately I haven't tried using the regular MH bulb all the way through so I don't have anything to compare... at the end of the day though if you get the conversion bulb then you save those extra bulb hours by changing the bulb half way through your grow...