Last time I answered this question I pissed off the thread starter.

HPS lack blue light. Blue light is used by Chloraphyl A. This is what the plant like to grow on (get bushy).

MH lack RED light. Red light is used by Chloraphyl B. This is what produces the hormones and sugars used to produce flowers.

Thus the short answer is:

If you use a MH for the entire grow you'll have strong healthy bushes with tiny buds.

If you use HPS for the entire grow you'll see some streching, the plants will have fewer leaves and branches and thus fewer bud sites. However the buds will be big and healthy.

If you use both the entire time you'll have more leaves, more branches, more bud sites and bigger buds.

Don't believe me? Read the lighting section.:thumbsup:

Good luck.