I just found the answer in the Overgrow FAQ....I would still like to know if its better to use an HPS ballast or MH ballast.

A Conversion bulb offers you the opportunity to switch the bulb in your Halide or Sodium fixture without changing ballasts.

Many cannabis gardeners prefer Halide light for vegetative growth and a Sodium light during blooming but a separate fixture is normally required. A Conversion bulb is a convenient option when you only have room for a single fixture but want the advantage of both Sodium and Halide lighting.

Please note:

High intensity discharge conversion bulbs are system specific. Sodium conversion bulbs are designed to be used in metal halide or mercury vapour fixtures only. Halide conversion bulbs are designed to be used in high pressure sodium fixtures only.