YMMV, but OneNote makes me nervous. I briefly played with it on OSX, and didn't see a good way to get notes out of it (e.g. for backup, recovery/usage in absence of OneNote). That is one thing that Evernote gets right IMO - the local store is just a bunch of html files + images, one directory per note.

For math - it's been over a decade, but I think I'd need a big device (ipad pro 12.9) and a pen. I'd want to be able to get a full page of notes on screen at once. That said, I'd be tempted. I've scanned my old notes for posterity and to save space, but it would have been nice to just capture them in digital form in the first place.

I'd also want a TeX stack on the device (editor+preview), but I'd write that if I had to. (I did pure plain TeX back in the day.)