I haven't used Mendeley since I started using Papers, about 3 years ago. Mendeley at the time seemed to have an uncertain future and less than stellar performance. I find the Papers experience overall much better designed, though the 3.0 release, status post Springer's purchase of Mekentosj, still has some warts. Specifically, I found the PDF interaction is much better and with 3.0 the sync and mobile support is great.