Hi Bartco! I can give some thoughts on this as I wrote the OneNote Guide.
Outlook will give you one central basecamp for the key components for your workflow: Email, Calendar, Contacts, Tasks, and Notes. OneNote will only replace Outlook Tasks and Notes. So you'll still need Outlook or something equivalent for Email, Calendar, and Contacts.
Here are some pros and cons that I see...
Outlook for Tasks:
Easy to turn an email into a Task
Easy to drag and drop Tasks on to the Calendar
In the same app where you are working Email and Calendar, so less toggling around
Sequential/linear design, for those who like to see things in traditional list format
Supports due dates
Notes app is extremely limited.
OneNote for Tasks:
More visually creative design for lists, for those who don't want linear format
Expandable list options (i.e. multiple Notebooks, tagging, color coding tabs, etc.)
No due date structure (but I explain options for that in the Guide)
A little more work to capture actionable emails into OneNote entries
Outlook and OneNote do integrate and sync, but it's not as streamlined as you might think. And I can see many people get tangled up in overusing this feature, particularly if they are trying to mirror everything in both Tasks and OneNote.
A couple of scenarios that could work would be to use Outlook Tasks for Project and Next Action lists, but use OneNote for project support and reference. Or, don't use Outlook Tasks at all and put all lists in OneNote. That's the scenario I lead with in the Guide, because I didn't want to make the Guide just about people who also use Outlook.
There is value in getting both Guides, if you will be using both apps.
Hope this helps!
Kelly