People say it is better because it has more blue spectrum in it, which makes the plants less tall and more stocky. Also, some say the extra blue in the light makes it form more crystals. In reality, it does simulate early to mid summer, and the HPS is more comparable to the end. The red in the HPS makes plants stretch more. I have found HPS buds are slightly larger, but the MH one's are denser, so it makes both almost equal in weight.

I have used both in a controlled experiment a long time ago and really there is not that much difference. It really is trivial and use which ever you have on hand at the time. Keep in mind the MH are some what hotter.

Hope this helps and good luck!

EDIT: Sorry I just realized the question was about the plants in veg. Both are very similar, but you will find slightly shorter stronger growth with a MH and a little taller plants with the HPS. Like I stated above, it is very similar so use what you have.

TGT