Patrons say patron. Always call people what they want to be called.
Ok and also,
I think patron most perfectly encapsulates the relationship between the library and the user, wherein the library is dependent on the goodwill of the user in order to continue its existence. I think the use of the term customer is detrimental, in that it sets up expectations for an entirely different relationship, one which runs counter to library ethics, one which exists primarily in a for-profit environment, where buying power determines worth. I really do believe strongly that the word patron is still at the heart of the library’s relationship to its users.
And just one more thing,
Patron is a gender neutral term, like actor.