My $0.02.
I am pretty new to growing, but I have used and tested many different light sources. My personal favorite source of light going into my current grow has been the Blue MH from eye Hortilux. Not the cheapest bulb out there but man it packs on the trichs. I prefer running this bulb all the way through in my HID lights. But then again I also prefer the spectrums with some more blue in them in the LED's. I think if you can run a "Blue" MH on top of the HPS you would get kick ass results, if you have that option I'd definitely take it. Now realize I am talking about a specific bulb not just any regular old MH.
They make the bulb in various different wattages:
http://growershouse.com/eye-hortilux-blue-daylight-super-mh-lamp-600w
Now if you were on track to use your MH for veg and then the HPS for flower then have at it, will give you a good base to compare later on. Not trying to tell you what to do just offer some advice.