Many times the stuff marketed as garlic powder actually is granulated. Real onion or garlic powder will be very fine textured like flour. If its coarser than that, you are most likely already using granulated. Freshness is another issue howeve, and anything you get from a reputable online spice dealer will most likely be pretty fresh. Also places like Sam's and Restaurant Depot will probably be fresh as well due to the volume that they move.
As far as granulated being better, that's true. The difference between the two is that granulated will be pretty much 100% garlic or onion, while the powder is usually made from husks and root ends as well. Granulated is just a purer product.