Claro in Honduras will tell you beyond all doubt that their card will function fine in Guatemala and in El Salvador. They explained to me that all if my unused data may be used in these neighboring countries, but that I will not be able to add to the Honduran card and will need to purchase a new card in each country to gain more time.
This is incorrect.
The moment I crossed out of Honduras into Guatemala my iPhone alerted me that the phone number had changed. Perhaps I could have done something if I understood the Spanish. However, there was no text message about this, so no way to know the message, issue, or fix. What I do know though is that the network name appears perfectly well, that I am told I need data roaming and that turning roaming on did nothing to give me data. Local-living ex pats tell me the Honduran sim cards do not provide service in Guatemala.
As I understand it, it only pays to purchase a Guatemalan sim card if you will be in Guatemala for a month. The fees are much higher hear than in Honduras.
NOTE: Remove your sim card from your iPhone (or iPad?)! If you do not, the notification messages about the network not being found or changing will (a) interrupt your typing and (b) kill your battery.