I was wondering the same thing.. well kind of.

I have a HPS ballast that can take an MH bulb. I was wondering whether I should pay the extra ~$50 for a MH bulb or just veg with the HPS...

I think you're fine vegging with that set-up you've got. You can veg with just HPS, but I've heard it's not ideal... You might even be able to veg with just those fluorescents, especially if you're concerned with power. I think fluoros work pretty well for veg, just not the best for flower..