Experience...

I have never seen an Mh bulb preform like an HPS...
In the ideal world you would veg inder MH and Flower under HPS...

From what I have witnesses... an HPS is the way to go if you have only one BULB.. they are not even close...

The most important factor is proximity..... the farther the light is from the canopy the more the plants stretch...

THERE is NO reason at all NONE.. to allow stretching....
Any serious grower who is INTENT on getting the most from his light will keep the light no further than 12 inches from the foilage...

: )

i love you