MH is better for veg, and vegging takes up far less room and does not require less lighting square footage, so use the 400w with a sunmaster warm bulb for veg(it can be added to flower phase, because you can't have too much light!)
Use the 1000w with a hortilux bulb for flower...your grow will be bigger, requiring more square footage to cover and you will be set...imho...hope this helps...
I have found that since I replaced regular 400w bulb with sunmaster warm deluxe bulb,
my plants have fluorished, like I've never seen...I would not use HPS conversion in my MH light...If you have both types MH and HPS lights, then you are blessed...I have both and I wouldn't give up the verstility of having both...jmho...