I must preface this by saying I haven't smoked a lot of either, but I have smoked both and very much enjoyed both. The J21 had just a bit more of a sweet, spicy flavor (nutmeg, allspice type) where as the 7th had just a bit more earthy, cocoa, coffee notes. They're both fairly peppery. If I had to choose one over the other I don't know that I could.
Dan