My perspective is that Ammonia is a storage mechanism for hydrogen. The question is more like cryogenic vs compressed air vs chemical (such as ammonia, or hydrogen peroxide). A bigger question is production: solar, hydro, and wind are probably cheapest for electrolysis. Thermochemical production may make sense for nuclear or geothermal based power. Maybe Iceland becomes a big gas station for transatlantic shipping, based on geothermal energy?