With slide decks - one may change an individual slide an assess if it was better or worse; however when one creates unique artifact each time it will naturally mutate. Some times for the better, sometimes for simplicity, and at times for the worst. Yet with a flip chart artifact, if it is unclear, you can just pull out a marker and fix it in real-time. And you may learn what will improve the artifact the next time you use it.
Some people don't like the high amount of time required to create posters for a course each time. Yet it is a great time to think about the class. I believe this preview of the event to come is a great form of future visioning. It is a practice session, much like an olympic athlete that visions their event going well minutes before they start.
Here's a MS Photosynth of a training with lots of custom posters. They get a little better each time. I encourage you to try this technique.
Why I use Flip Charts not PPT Slides