In a new tank, the ammonia one will work great, to show your cycling. But once the tank is cycled, then it's totally useless because you shouldn't have any ammonia anyways. It will always read 0ppm. And as for how long they actually last and how accurate, not sure.
55G Filstar XP3 - 16" Fire Eel
75G Medium Planted - Filstar XP3 Low Light - Established Feb 2006