One of the killer features is the multi-platform support, including web browsers, in combination with synchronization across devices and the Google Docs plugin. Makes working on a paper across multiple devices super easy.
One of the other things going for Mendeley is seamless sync between an iPad and a desktop. Their cloud limit is 2GB free storage.
Though Zotero has Zotfile, it’s a hassle to set up tablet sync with an iOS device.