Evernote, OneNote, or Google Keep? The answer is all three
The first tool I used was Evernote. It served exactly the function that I wanted. It is basically a prosthetic memory. There are all sorts of documents and records that have some sort of value and that I don't want to forget, but that didn't have a good home. Evernote serves that role. I can save documents locally or clip them from the web. I can assign various tags to facilitate projects and re-finding. The documents are available from both my desktop client and the web.
Of course, the tool isn't perfect. Tagging is great but as we know from the academic PIM research tags aren't completely effective. It's a wetware problem. Users forget what the tags mean because they are generated to meet local constraints. Furthermore, Evernote breaks formatting on messages so that it is difficult to round-trip content into other Office applications. Regardless, Evernote serves the role of a memory-prosthesis quite well.
There are few great features in Evernote. I particularly like the ability to email documents into the repository via an email address. In this way I can essentially BCC myself key communication to keep an archival record.
Evernote is good for many things but sometimes I'm looking for a tool that offers better support for personal workflows. The primary applications that I use on a daily basis include Word, PowerPoint, Excel and -- most importantly -- Outlook. There are a few things tasks that are highly repeatable and require tight integration with Outlook. Consider, for example, client meetings. Each of these meetings requires minutes and some sort of formal follow-up. Outlook basically gives me a simple workflow capability. From each invitation I can generate a OneNote note that captures document details and enables me to generate very basic case notes. I can then use notebooks to essentially capture the status of these notes (completed, in progress, etc.). These notes are formatted perfectly for re-purposing into the other Office applications. Furthermore, I can apply "to do" tags that work with my task management tools in Outlook.
I find OneNote for less helpful for capturing external documents, applying subject tags, etc. Indeed, in some cases I actually end up dumping content back into Evernote if I want to archive it. For example, I might use OneNote to generate client communication (largely because I can reuse content from similar correspondence) and Exchange to send the correspondence (I could do it from OneNote but I need to redact superfluous details) but I BCC my Evernote account so that I can archive the details. NOTE: I might also have to copy that content into other case management tools like CRM systems, etc.
So OneNote and Evernote can live together quite effectively. But what about our third option? What is Google Keep?
With OneNote and Evernote I found that I no longer carried around my notebook with numbered pages. Instead, I carried a tiny notebook for lists, etc. This is the application for which Google Keep excels. You can create lists and reminders. It integrates nicely with Google Applications and -- for Android users -- Google Now. The reminders are particularly beneficial. On Android, you can set location reminders so, for example, you can attach grocery lists to the location of the store. When you walk through the door, you get a reminder.
For me, it's not an issue of Evernote OR OneNote OR Keep. It's really about finding the appropriate use cases to support Evernote AND OneNote AND Keep.