There is a difference. It's all about essential oils and volatile compound content. More = better/more pungent spices. Powdered spices, by their very nature, have less of these compounds because of the higher level of mechanical processing necessary to manufacture them into that form. Granulated spices on the other hand, have a much better chance of containing more of their original EO and VC content. Please keep in mind however, not all spice manufacturer's spices are created equal. There is a vast and dizzying array of levels of quality out in the market place. Unfortunately, virtually all of this grading takes place behind the scenes at the manufacturing level and this information never makes it to the consumer.
This is why I demand that my spice supplier/blender only source their spices from suppliers who routinely test and report their spice's EO and VC content via MDS sheets or other publications. I monitor these values to make sure my rub products are made with consistently the very best spices available.
However, as a general consumer there are things to look for, and unfortunately price isn't always the best indicator. It can help, but it isn't 100%. First buy granulated spices where ever possible. Don't buy salt/spice combos like garlic salt or onion salt. These are almost always made with inferior grades of spice. Instead, make your own from the individual components. Look for a deeper richer color and/or more pronounced flavor and aroma. And finally, look at how the spices are packaged. The more air tight the package, the better. If you can pick up a spice from the store shelf and smell the spice through the container, walk away. Oxidation from free oxygen exchange through the spice container is the primary cause of dull and lifeless spices, regardless of their original quality.
Hope this helps.
Oakridge BBQ LLC