I am not going to tell you which note app I run. The app is the least load-bearing part of this, and if I name it you will argue about the app instead of the decision. So I will do the more useful thing and show you how I decided, one axis at a time, including the option I threw out and the reason I threw it out.
The decision was not "which app." It was a layout question: should capturing a thought and keeping a thought happen in the same place, or in two places tuned for two different jobs? For years I assumed one place, because one place looks tidy on a whiteboard. I now run two, and the thing that moved me was a diagram I half-remembered from an undergraduate architecture course, sketched on a napkin at a coffee shop in February.
The shape of the problem
Here is the diagram. A CPU does not have one kind of memory. It has a hierarchy. Registers at the top, then L1 cache, then L2, then L3, then main memory, then disk. Each level down is larger and cheaper per byte and slower to reach. L1 is on the order of tens of kilobytes and answers in about a nanosecond. Main memory is gigabytes and answers in around a hundred. No single technology is fast and large and cheap at once, so the machine refuses to choose. It builds levels and moves data between them.
