I take notes on the lowest-tier Strathmore 9x12" spiral-bound sketchbooks. I strongly prefer totally blank paper for math and diagrams. The paper is plenty high-quality, you can tear out pages without ruining the appearance, and they are cheap.

However, I don't view those notes as a permanent record. Anything really important gets transcribed to LaTeX (math) or text files (TO-DOs, etc.)

The transcribing takes extra effort. I'm not sure it's worth it, but it's a good opportunity to distill things to their essence and double-check logic. My future self doesn't need to wade through as many dead ends when looking back later.