Do the cardstock. It'll give you a good idea of how the basic helmet is put together and how the pieces mesh. Then if anything, you can use THAT as a reference for cardboard. You would have a 3d picture basically you could spin, rotate, etc, and cut cardboard shapes to match what you need to complete the helmet. Look around on the forums for some other people doing cardboard if you want to go that route. Most put progress pictures you can kinda see how they do it.
This thread is a good example of one. Royal Viking is making a real nice cardboard helmet, you can follow the thread and see how he's doing it. Basically cutting piece by piece and gluing it on however he feels it needs to go.
With cardboard you need to use your imagination and cut out your own shapes, with pepakura, its all setup for you, just glue it together.
