hey man if you only have this 2 type of bulbs and want to use them for veg's and flower then i would say that you should use the 400w MH bulb for veg's becors MH is like a summer day spec , and the 600w HPS bulb is like a late summer day so the 600watt is perfect for flowering stage and the MH 400watt is perfect for veg's your plant's then run a cool tube to get the MH closer to your clones and seedlings they use MH in the most of the medical grow for shop's and big grow's becors they grow abit faster under MH then fluorescent and Cfl's. but its cheaper to run fluorescent and Cfl's that's what i do think