Generally , the content Site Page within the Site Pages library is a wiki page.
But what is differ from the
wiki page library and
site Page library that the
site page library contains all pages including the home page , and when you add a new page to your site , by default it's created below site page library.
Regarding a wiki page library it's an interconnected set of easily editable web pages, which can contain text, images and web parts.
it's used when you have mostly a need for rich text content; for example, text, tables, links, and images. You can still insert Web Parts or use no Web Parts at all. The HTML content in wiki pages is also subject to versioning if versioning is turned on in the library.
While wiki pages are flexible and very customizable, one of their drawbacks is the lack of metadata that’s automatically available in them. As you add content to the page, the metadata doesn’t automatically follow. That, as it happens, leads us nicely to Publishing pages.
In my opinion, I think the compare between
wiki page and
web part page is more useful than comparing with site page that is a wiki page, so I suggest to check also Difference beween wiki page, application page, web part page and publishing page