I use a 400w MH for my Veg Stage and a 400w HPS for flower. I have never heard of someone using MH during flower because its color spectrum is for veg growth. Whenever i switch from MH to HPS i Notice a huge difference in type of growth, will notice many bud spots starting within days. You could use the MH for flower stage im sure but like someone said above, the yields might not be as good.
So basically: MH=Growth
HPS=Flower