As several of you have already stated, North Korea is most certainly more of a threat than Iran, much more. Their leader is young, and may not think completely logically. They are weak now, but they will get stronger.
As for politics involving such a war, there's not as many problems as people think. Technically the US and South Korea are still at war with the North, we're only in a ceasefire, which North Korea has violated on several occasions, they've even attacked South Korea. Surely our image can't be hurt by responding to an attack? Besides, since then several other countries *cough*Russia*cough*France*cough* have been in wars.
As for China, they have more to lose by supporting North Korea than they do by sitting it out. I doubt they'd get involved. The only way I could see them getting in is by joining the West in attacking the North, so they could have some say in the post-war.
As for Iran, I personally think they can be ignored. Even if they develop nuclear weapons I don't think they're as likely to attack as the North Koreans.
The only con of such a war would be money. However, as far as that goes, I think North Korea would be far less costly as the wars in the Middle East. For one, the new United Korea would be governed by Koreans, and the people in North Korea could be pacified easier than Iranians. They believe their leader is a god, and if they see him defeated it would likely change their mind.