Thank you so much for all the answers and discussions!
I didn't expect to get this much detailed knowledge and experience sharing.
I had almost decided on the Herzberg Festival, but now I am considering going to Molde instead.
Coming from south norway, the distance is almost the same. But i haven't been that far north in about 10 years, so it's about time.
On the other side, I'm not interested in much else than Motorpsycho in Molde.
The Herzberg festival sounds like a lot of fun, and can be combined with a trip to the Netherlands or somewhere else in Europe, but they only play for 45 minutes..
Molde is probably a better, longer concert, and a lot cheaper too, considering travel expenses and such.