If you set up for a continuous cycle, ie, you have batches of plants in several stages of grow at any given time, you won't need conversion bulbs, since you'll always have the same types of lights going. And really, that's the way most professionals do it. It maximizes your space usage.

If you get digital ballasts, you can use regular MH or HPS bulbs in them, and you won't have to bother with expensive, short-lived conversion bulbs. Plus you'll be more energy efficient and you'll have less noise.