I stumbled on this PubMed publication(!)
It is outdated but good enough to give you the gist and an overview of how things stand today.
All four of the tools reviewed here are usable for standard reference manager functions, and each has strengths and weaknesses. For example, in our testing, Zotero’s browser add-on was the easiest to use and captured data more accurately than the other add-ons did. EndNote offered the most choices for searching databases within the tool, and Zotero generated the most accurate bibliographies. Each also offers unique features that may be especially valuable to certain populations (e.g., RefWorks’ integration with Google Docs, Mendeley’s social networking functions). Often, though, the best choice for a given purpose may be determined by factors other than the functionality of the applications themselves. These factors include cost, support provided by institutions, research needs, familiarity with a product from previous experience, and accessibility for the research team members.
via Choosing the Right Citation Management Tool: Endnote, Mendeley, RefWorks, or Zotero