1. N. Korea has an instinct for self-preservation. They already have nukes. Iran has a fanatic leadership which believes it is on a mission from Allah to rule the world. They have threatened to destroy Israel directly. The Iranians have directly sponsored terrorists. North Korea wants to conquer S. Korea intact so they can bleed its riches. Iran would be happy turning Israel into a lake of glass.
2. The Republicans have long been arguing that N. Korea is part of an Axis of Evil that needs to be stopped. We've already blown stopping N. Korea from getting nukes. N. Korea is probably the least scary of the bunch because Kim Jong Un is more interested in preening and preserving his privileges than he is in ruling the whole world. The Ayatollah, on the other hand, believes as a matter of religious doctrine in a Caliphate that will one day rule the world.
3. Iran, so far, does not have nukes. It is important to keep it that way because Iran has a history of arming terrorists.
You can't trust them is not naive. What is naive is to assume a government that regularly leads group chants of "Death to America, Death to Israel" doesn't mean what it says.