I have an ongoing (and friendly) dispute going with at least one person in Evernote’s User Forums (Hi PE!) as to the best way to classify your data. I have a personal method that uses both tags and notebooks - and a third option: titles - to make my life a little easier.
As I also have nearly 60,000 notes and 300+ notebooks, and an ambition to be the Laziest Person In The World, IMHO easy is good!
I don’t want to argue about what’s "best", or why this method works (for me) - you may prefer, or need something completely different. That’s perfectly reasonable and the choice is entirely up to you. After 40 years or so of managing commercial and personal databases, this is just mine.
I regard Notebook names as being a branch of the Tag family. They’re not significantly different, just another way to split up your data into manageable chunks. I prefer to use the subject or focus of the note as an indicator of which notebook should be used - anything to do with my Amazon books and other purchases goes into a notebook called "|Amazon". Anything to do with the estimable automation tool Zapier goes into "|Zapier"; and all places in-between.
All suppliers, customers, personal connections, projects and applications that have any form of clip, email or scanned document, get their own Notebook. - And surprisingly, I have less than 400; I actually thought it would be many more than that.
"Ah," I hear you say…"but what about things like projects where you write to a colleague and refer to a Zapier process (a ‘Zap’) in connection with it?"
-And that’s where a little personal judgement and Tags come in. The reason for that email was the |Project, so it gets filed there: but to involve the |Zapier and |Person notebooks, I’ll use tags: specifically "x|Person"
Now the perceptive amongst you will have noticed the prepend/ prefixes "|" and "x|". That’s just to make it easier to find and move things. Do a search in your own data for Amazon and you’ll probably see what I mean. That name gets mentioned a lot in places you don’t necessarily expect it - sometimes in very small print! A review of War and Peace might include "this book is available from Amazon"...
The more notes you have, the more likely that any word will occur in many more places than you actually need to find it. Using a ‘unique’ word means "|Amazon" can only be the notebook. The tag is slightly different because "|Am" finds the notebook and "x|Am" finds the tag - both in my searches and when I’m assigning or moving notes.
OK - now Titles: my titles follow a standard format. yyyymmdd - type - source - keywords. So a mail to Amazon last week about a new laptop might be "date (of the mail) - email - Amazon - Dell laptop"
…And there you pretty much have it. Looking at my (vaguely embarrassing) Amazon purchase history just requires looking at the Notebook, sorted in Title order - the latest stuff at the top. No searches required.
I have a saved search in favourites to connect me with notes sorted in creation date order, which gives me all the latest notes from all notebooks.
Using Evernote's advanced search syntax I can find anything else by date, subject, tag, type (receipt or letter forinstance) or whatever. False positives are sorting errors I can correct, or (more often) just search errors I can easily rule out.
Just for completeness, there are a couple of ‘advanced’ tips below involving some Fremium apps which may be of interest - but you can skip those for now if all you wanted was the notebooks vs tags summary.

Advanced Tip #1
Just to add the extra spice I subscribe to an external app "Filterize" which - based on keywords, email addresses and the occasional key phrase, can take any random forwarded email, web clip or scanned document from my Default folder and apply the necessary tag(s) whist also moving it to the correct notebook. Mostly.
Adult supervision is sometimes required - plus some judicious editing of the key words that are listed in a special note as a table of keyword(s) | tag(s) | notebook.

Advanced Tip #2
I also subscribe to an organisation app "Workflowy" which has (allegedly) infinitely collapsing bullets. I’ve copied my notebook structure there and saved the tags combinations which should apply as "tag1,tag2,tag3" (ie in one copy/ pastable string) so that any notes I create are a couple of keystrokes away from the (correctly spelled and formatted) name and tags.
Workflowy helps make it quicker to navigate the set-up and apply the necessary details where required. And (sorry Evernote) it’s a lot faster than using Evernote’s Notebooks page.
This app also includes the option to ‘Mirror’ a bullet - so that it appears in more than one place in my list and can be edited from anywhere. Very useful to keep URLs and contact details up to date.

Advanced Tip #3
-And in wondering where to publish this so most users will have access I decided to revive my otherwise languishing blog - run from an Evernote notebook managed via another external app "Postach.io" to which I also subscribe…