There's differences not only between strains, but also in how the plant is raised and matured. As far as I know, if the plant is allowed to ripen longer, which tends to be longer than the typical grower would like to wait, then the plant should have more narcotic, pain-relieving properties. Although I don't know about this from experience, so others will know more about this. But basically, if the resin crystals on the bud are amber or red for the most part, it may provide more pain-relieving qualities.