I do think there are differences between catnips. One will be what is included...some catnips included a lot of stalk plus leaf, the better quality catnips contain more leaf and flower with some stalk, and the finest of them will be a fine grind with no stalk at all.
Potencywise, any catnip with a higher mix of the flowers (or buds) will be strongest, as the active ingredient in catnip is contained in the essential oil, which is in highest concentration in the flowering buds.
Ways to give catnip...you can buy a catnip oil in water spray (my favorite due to its purity) or one of other catnip sprays, and you buy the flowering buds on the stalk (just remove a bud or two, crush between your fingers, then sprinkle atop a cat toy or bed or on the floor.)
You can even get catnip compressed into non-messy pellet form - my experience is not too many cats will eat these, but they make a wonderful refillable cat toy filler (albeit they make for lumps!).
I agree that organic is better - most cats will consume some of the catnip, why introduce any chemicals?
anyhoo, hope this helps, catnip, as you can see, is a passion!