A good tour guide can make the difference between an ok tour, and an amazing tour. I've had some tour guides spout off facts that were more myth than reality, and others that had clearly studied up on the areas and history. As a photographer, I'm always communicating up front with any tour guides and agencies that I am a photographer, and looking to take photographs of certain things. It helps a lot if the tour guide knows what you're looking for out of the tour.
In North Korea, as you're generally in more of a group of 20 or so tourists, the situation is a little different. Your itinerary isn't going to really change whether you're on a private tour or a group tour, so you're best off taking the group tour option, saving some money, and having the company of some other people around.
Overall I found the tour guides in North Korea to be quite good. Well versed in history, and able to manage the challenges of taking foreigners around who generally have a very different outlook on life than North Koreans. Some were better than others of course, but either way you're in for a treat. This particular tour guide was one of my favorites, but sadly she stopped being a tour guide after getting married.
I'll never forget my third trip to North Korea where I ran into a tour guide from my second trip, and he immediately recognized and remembered me! I don't think that would happen almost anywhere else in the world, but what an amazing place to tour and see life under different conditions!