I was advised a few core exercises by my physical therapist to improve my postuer after a lumbago, among them the plank. As was explained to me in this answer, I should train until I hold about 60s in one go and then add variety to increase intensity. The alternative would be to keep working on longer holding times.
I bore easily, so I will probably prefer the first option (once I get there). But what is actually better for me and my posture.
p.s. I'm asking here and not the professional because the question only occured to me now, and I don't see the guy anymore.