The purpose of citing is to help your reader locate the sources that you use in your paper. Because some online sources are behind firewalls, licensed, or otherwise publicly inaccessible, the reader may not be able to access the source URL.
Style guides provide recommendations on which URL is preferred:
Follow these guidelines for including DOIs and URLs in references:
Include a DOI for all works that have a DOI, regardless of whether you used the online version or the print version.
If a print work does not have a DOI, do not include any DOI or URL in the reference.
If an online work has both a DOI and a URL, include only the DOI.
If an online work has a URL but no DOI, include the URL in the reference as follows:
APA: 6th
Use DOI when possible. Section 6.31 states:
"In the ephemeral world of the web, article links are not always robust."
"Developed by a group of international publishers, the DOI System provides a means of persistent identification for managing information on digital networks."
"The DOIs in the reference list function as links to the content you are referencing." See this APA Style Blog Post: What to Use—The Full Document URL or Home Page URL?
Section 6.32: Providing Publication Data for Electronic Sources, pp.191-192):
Provide the DOI, if one has been assigned to the content. Publishers who follow best practices publish the DOI prominently on the first page of an article. Because the DOI string can be long, it is safest to copy and paste whenever possible. Provide the alphanumeric string for the DOI exactly as published in the article. This is not a style issue but a retrieval issue.
When a DOI is used, no further retrieval information is needed to identify or locate the content.
If no DOI has been assigned to the content, provide the home page URL of the journal or of the book or report publisher. If you are accessing the article from a private database, you may need to do a quick web search to locate this URL. Transcribe the URL correctly by copying it directly from the address window in your browser and pasting it into your working document (make sure the automatic hyphenation feature of your word processor is turned off.)
Chicago
Section14.9: Permalinks and the like: "When a URL points to a location that requires a subscription to a commercial database (e.g., through a library), it may be better to name the database instead (see 14.11)."
Section 14.11: Library and other bibliographic databases. "For a source consulted via a library or other commercial bibliographic database and available only through a subscription or library account, it may be best to name the database in lieu of a URL. Even a URL recommended for such a source (see 14.9) may lead a nonsubscriber to a login page with no information about the source itself. If in doubt, test the URL while logged out of the library or database; a URL that leads to information about the source, if not full access to it, is safe to use. A URL based on a DOI, which will always direct readers to information about the source, if not full access to it, should be preferred where available (see 14.8). For more information and examples, see 14.161 (books), 14.175 (journals), 14.215 (theses and dissertations)."
MLA
Pg. 48: "The publisher of a work on the Web can change its URL at any time. If your source offers URLs that it says are stable (sometimes called permalinks), use them in your entry. Some publishers assign DOIs, or digital object identifiers, to their online publications....When possible, citing a DOI is preferable to citing a URL."
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