John Mueller while replying to a tweet today said it is best to be consistent with your URL. Either choose to use a slash after URL or not use one. Refrain from using both formats if possible. When asked what happens when a site has thousands of pages with two versions of URL, he replied.

use slash after URLs or not.

John Mueller, a while back had talked about trailing slashes in URLs. He had said, “the slash after the hostname or domain name is irrelevant, you can use it or not when referring to the URL, it ends up being the same thing.” John said that it has nothing to do with Google or SEO but rather how a website works. 

To explain his point he posted the below example in which, works fine shows a 404 error.

Either way, John has suggested to stick to one version and use it for canonicalization, redirects, etc.