I didn't see your last post before I posted mine. As far as the same bulb for grow and veg I highly discourage it. It will work and you will get weed, but not nearly enough for yor effort. If you go this route, make sure the emphesis is on the flowering (more red spectrum like HPS), because after all, that's the important part. Anything will work for the veg stage, just not as well as MH. There are switchable ballasts, and their are conversion bulbs. Either of these would be a much better choice than one bulb. I think (not sure) all digital ballasts allow you to use conversion bulbs, so they are similar to a switchable ballast.

I think if I were to buy only one HID ballast that were not conversion or switchable, I would buy it for flowering and use blue spectrum flouros for vegging. You would probably get about the same results for much less electricity. There is a formula or two to figure this out somewhere.