One way of looking at the collections is to rate them by the level of availability. For example, the Library of Congress is undoubtedly the largest library in the U.S. but how available are it contents to the general public (including all the genealogists)? Not very. The huge collection in Washington D.C. and surrounding areas is not generally available outside the library. A search of the Library of Congress website on the term "interlibrary loan" returns no results. Likewise, the Family History Library has limited provisions for sharing its materials outside of the library itself in Salt Lake City, Utah.
In the Wikipedia article on interlibrary loan, sharing of items between libraries is qualified as follows:
Loan requests between branch libraries in the same local library system are usually filled promptly, while loan requests between library systems may take weeks to complete. However, if an item is rare, fragile, or exceptionally valuable, the owning library is under no obligation to release it for interlibrary loan. Some collections and volumes, especially bound journals and one-of-a-kind manuscripts, are non-circulating, meaning that they may not be borrowed. Books may be delivered by mail or courier service. Photocopies may be faxed or scanned and delivered electronically. Urgent requests are placed if the item is needed right away, sometimes for additional fees. Public libraries do not usually offer urgent service.Contrast this with the Allen County Public Library which states, on its website, "Interlibrary Loans: The library uses ILLiad software to expedite interlibrary loan service to ACPL patrons. You need to register with ILLiad the first time you use it. Subsequently, logging on with library card number and password will connect you to ILLiad. You can make requests, track your requests, view your request history, and receive your articles electronically." ILLiad is the electronic system used to request an item through Interlibrary Loan. The name ILLiad can be thought of as an acronym for InterLibrary Loan internet accessible database.
So from the standpoint of availability to those living outside of the normal service area, the public libraries like the Allen County Public Library and the New York Public Library offer some advantages in obtaining copies of materials in their collections through interlibrary loan.
What about electronic access? All of the large libraries have extensive online catalogs of materials. For example, the Sutro Library which claims to have the largest genealogical collection west of Salt Lake City, is part of the California State Library system and its collections can be searched in the California State Library Catalogs.
I decided to try an experiment. I would look for a rather obscure family history book, one I know is in a number of libraries from my own family. The book is:
Overson, Margaret Godfrey Jarvis. George Jarvis And Joseph George De Friez Genealogy. Mesa, Ariz: M.J. Overson, 1957.
Guess what? A search on WorldCat.org says that both the Allen County Public Library and the Library of Congress have the book. At the Library of Congress the book is available at the Jefferson or Adams Building Reading Rooms. The item is also available at the Allen County Public Library and probably is available on interlibrary loan. See http://acpl.aquabrowser.com/?itemid=|acpl-bibs|ocm18555441 By the way, the book is also in a number of other libraries including the Arizona State University Library right here in Arizona. (Assuming I didn't already have a copy of the book of my own).
So, its nice to know that these huge libraries have the books or other records, but it is also nice to know if the materials are available outside of the physical location of the library.
Next, what about online access?