Given their disparate functionalities, you may be better off choosing one of Evernote and OneNote and separately Mendeley or ReadCube. Personally I don’t think the four are broadly interchangeable though they may be easier to work with in pairs for their separate functionalities. While I loved Evernote, I have generally gone “all in” on OneNote because it’s much better integrated with the other MS Office tools like email, calendar, and my customized to do lists there.
Another interesting option you may find for sorting/organizing thousands of documents is Calibre e-book management. It works like an iTunes but for e-books, pdfs, etc. If you use it primarily for pdfs, you can save your notes/highlights/marginalia in them directly. Calibre also allows for adding your own meta-data fields and is very extensible. The one thing I haven’t gotten it to do well (yet) is export for citation management, though it does keep and maintain all the meta data for doing so. One of the ways that Mendeley and ReadCube seem to monetize is by selling a subscription for storage so if this is an issue for you, you might consider Calibre as a free alternative.
I’ve been ever working on a better research workflow, but generally prefer to try to use platforms on which I own all the data or it’s easily exportable and then own-able. I use my own website on WordPress as a commonplace book of sorts to capture all of what I’m reading, writing, and thinking about–though much of it is published privately or saved as drafts/pending on the back end of the platform. This seems to work relatively well and makes everything pretty easily searchable for later reference.
Here are some additional posts I’ve written relatively recently which may help your thinking about how to organize things on/within your website if you use it as a research tool:
I’ve also recently done some significant research and come across what I think is the most interesting and forward-thinking WordPress plugin for academic citations on my blog: Academic Blogger’s Toolkit. It’s easily the best thing currently on the market for its skillset.
Another research tool I can’t seem to live without, though it may be more specific to some of the highly technical nature of the math, physics, and engineering I do as well as the conferences/workshops I attend, is my Livescribe.com Pulse pen which I use to take not only copious notes, but to simultaneously record the audio portion of those lectures. The pen and technology link the writing to the audio portion directly so that I can more easily relisten/review over portions which may have been no so clear the first time around. The system also has an optional and inexpensive optical character recognition plugin which can be used for converting handwritten notes into typed text which can be very handy. For just about $200 the system has been one of the best investments I’ve made in the last decade.
If you haven’t come across it yet, I also highly recommend regularly reading the ProfHacker blog of the Chronicle of Higher Education which often has useful tips and tools for academic research use. They also do a very good job of covering some of the though in the digital humanities which you might find appealing.