I'm currently using a notebook, but I find it very limiting in the amount of information you can write down. It becomes rather difficult to maintain structure or add text later on. Pen and paper works best for quick, abstract notes in my experience.

I still use pen and paper but I find that they’re best used as a scratchpad where the stuff you put in there eventually find their way in a TODO app where you can re-organize them or add more details as they are uncovered.