I use both. I have two 150 watt HPS one on each side and a 400 watt dimable ballast that use both type bulbs in the middle.
I use just the MH for veg then turn the MH to 50% and turn on both HPS for the first 2 weeks of flower, gradually turning the MH to 100%. Then I switch bulbs and use all HPS up until the last 2 weeks of flower, then I put the MH back in until harvest time.