i've been using mh's for my entire grow for like the past 3 or so grows after talking to someone on another forum about sunpulse mh's..
the sunpulses come in various k ratings, much in the same way the cfl's do... you just buy the correct k rating for w/e phase of growth you're in at the time.. ie, i use the 6 k for veg, and the 3 k's for flowering.. they even offer a 10k for the last two weeks of flowering, which i have, but have yet to run it yet..
like has been said, mh's have light in the uv range, which is supposed to increase tric production.. it's believed that trics are used by plants kind of like a sun tan lotion... the more exposure to uv lighting, the more tric production on that plant, or that's the science behind it although i'm not 100% about it tbh..
it's also said that the you get a fuller flavor profile from using mh's over hps bulbs.. something about the way mh's effect the terpenoids in cannabis..
overall, like i said, been running the sunpulse mh's for a good three of so grows now, and i have to say i'm impressed.. i'm loving them.. the blue dream i just ran has to be some of the stunkiest bud i have grown to date.. is it all the mh's or do i just kick ass at growing, lol, idk, but what i do know is i'm not going back to hps anytime soon.. it has been said though that hps bulbs will increase your yields while mh's will increase just about everything else, ie, tric production, flavor, etc, etc..