I would get a switchable ballast, then the only thing you have to get is 2 diff bulbs. Depending on what wattage system you get, that would be under $100 for the extra bulb. Then you can run your MH for vegging, and once you switch to 12/12 change bulbs to HPS and keep the same system. I mean that beats having 2 diff systems for each bulb.
Otherwise if you are going for just one, then at this point ide say HPS... but seriously it's worth the tad extra for the one extra bulb..