This is not strictly mathematical question but related one. In university I made notes on paper but notebooks got lost and I can not drag box of notes wherever I go. So I switched to making notes in txt files but I can not write nice looking formulas in txt files (or markdown). Then I switched to google docs It is wyswig tool which is very nice but it does very limited support for latex. Of course I could use latex and generate pdf but it is no wyswig which means I need two files: one to edit other to see what I did. So I wonder how you solved this problem.
I've been using Word with its Latex Environment, as it was the best compromise between speed and capabilities that I've found.
I'd say its Latex environment is an upgrade to what Google Docs can do, as it's basically a limited version of Latex (with quite a few bugs that will probably never be fixed), but it keeps the interactive style (you can edit the formulas both as latex-code or go in the displayed formulas and change stuff).
For my private notes, using Word this way suffices. What you get is not nearly as neat as what would be possible with Latex, and you'll definitely hit walls (but at least I don't hit them often enough that it would make me switch). But the WYSIWYG-nature makes it quite fast to get the notes to look "almost" as you want.
If you don't care about the possibility to search in your documents or to edit them, you can also take handwritten notes and then digitalize them. Using a handy camera it should be quite quick to turn a note digital, and there are blocks that come with special apps that automatically take care of cropping and rotating.
Yet another option would be graphic tablet (+ wireless keyboard). In terms of taking notes, I doubt there's anything that can measure up to the speed of this combination, though getting a good graphic tablet might be quite an investment.