My guess is that Florida has the jurisdiction, since that’s where the crime took place. John would be arrested in NJ and extradited to FL. NJ might have jurisdiction over seperate conspiracy charges though.
IIRC, both have jurisdiction. If even one predicate act to a conspiracy vests a forum with jurisdiction. As a practical matter, with both Florida and New Jersey having jurisdiction, Florida would probably end up prosecuting the matter, because they have the most significant interest in the ultimate crime.