More free cloud storage than Zotero (up to 2 GB or nearly 700 articles).Search tools based on what you have already added to your working citation library.
Mendeley is available in almost anything, including a Chrome web extension, bookmarklets for other browsers, and a mobile app.
The web and desktop application have a slick interface that is great for people who are not as computer-savvy.Less free cloud storage (300 MB, or about 100 articles).More organization features with more nesting folder options and unlimited private groups for no added cost.