: What's better for search engines? Doesn't matter. What does matter is: What's better for users? A modern search engine shouldn't rely on strange rules of what is a word separator and what

What's better for search engines?
Doesn't matter. What does matter is: What's better for users?

A modern search engine shouldn't rely on strange rules of what is a word separator and what is a word joiner or whatever. Modern search engines can deal with typos, apostrophes, punctuation, etc. Figuring out which characters separate words shouldn't be much of an issue.
I disagree with the people who claim that hyphens are better because they are natural word delimiters. They kind of are, but kind of aren't: they make words semi-attached or semi-delimited. The natural word delimiter is %20.

But the above statements are irrelevant. The URL shouldn't be important anyway.

How important are the keywords in the URL?
They shouldn't be important, if they are there's obviously no content on the page.

URLs aren't very visible for humans: links may have anchor text instead, it's not shown on the page and it's not shown on the browser tabs.
The <title/> and the main heading are more visible and usually contain the same keywords anyway making the keywords in the URL redundant.

How important is it for humans?
It depends.

From a search engine point of view: not at all, the user only needs to enter a search query and click on the snippet with an interesting title and description.

But visitors come from other places too. In some cases, there's a nice anchor text instead making the "URL quality" irrelevant, but there are cases when it does matter.

Quick&dirty copy-pasting/sharing of the URL: no issue for the writer, but it does matter for the reader.
Needing to enter the URL manually. (Can't copy text from an image for instance.)

What determines the quality of a URL?

Length; You don't want address bars to scroll horizontally or URL only links to linewrap. And needless to say: it also takes longer to type a longer URL.
Word delimiters; It appears that most people agree on that hyphens are better.
Clutter; Eg: Unique IDs, filename extensions, weird URL parameters. These are difficult to remember.
Strange characters and syntax; An outdated example would be tildes (http://example.com/~user/), but URL parameter syntax is a bit strange too. Any uncommon character might be difficult to type for some people.
Safe characters vs Unicode; This is a two edged sword and deserves its own answer. But briefly: browsers mangle URLs, %c3%a4 etc is a pain in the ass to type, not every keyboard can enter the unsafe characters, possibly some encoding hell, but the keywords make sense for native speakers.
Length of the text; Consider the URL to be a form of a title, don't waste words mentioning the obvious and ignore grammar.

People will type something slightly different
Your webserver should be designed to redirect recognized non-canonical URLs to their canonical version. It's up to you to decide:

http vs https
www vs no www
trailing slashes vs no trailing slashes

But your server needs to accept and correct all of them.

A 404 page with search results would be nice for the user. (Use the words from the URL as the search query.)