you cant use a hps bulb in a ballast designed for a metal halide.
and think you are making to big of a deal out of the hps/ mh issue, either one can be used in both veg and flower cycle so you do not neceassarly have to have both types.
and not sure I would even bother getting a 70 watt at around 6000 lumens you would probably be better off with 3 40 watt cfls or it would atleast give your plants more lumens.
a hps or mh is a better light but dont think I would even bother buying a smaller than 250 wattage in hps or mh.