If I was there, then I will choose these URL's, depend on my server configuration.
As you can see, the only different is, slash at the end of URL. If you are running Wordpress in nginx server, then you can easily serve your content without slash at the end of URL, just like stackexchange serve.
Technically first one is consider as file In UNIX system, while second one is directory. but choose whatever you like it. Just don't forgot to redirect between them if someone forgot or add slash at the end of URL.
Now, why I said, these URL's are good?
Because it is user friendly(No worry about how people link to you) + You can change your language/platform any time, may be in future Java/HTML/PHP/GO overtake to other language, then may be you will switch to other CMS, which is easiest for you.
So in the future(My assumption) all CMS and blogging platform allowed to use permalink without any extension name, so use these type of URL's, if you depend on new technology, because it does not effect in SEO, if you switch to another one. :)