At least as recently as 2005, diesel subs were the quieter option. There was that somewhat notorious story of the Swedish one that beat an American carrier group in a wargame because the Americans just couldn't find it. I'm sure there have been developments in the equipment and methods since then - it was 18 years ago, after all - but it's still notable enough that the Americans leased the sub from Sweden for a couple of years to practice against it.
That said, the Swedish sub in question was packed with cutting-edge (at the time) stealth features. I suspect North Korea's fleet is not.