J-15 is made of walnut back and sides and J-45 is hog back and sides. The necks are made of different woods too, J-15 is maple and the J-45 is hog. That is all I know the experts can tell you more. I'm sure they are giving the J-45 more attention than the J-15 due to the $700 or so price difference.
Why is the J-15 with a natural top $150 more that the same model that is painted?