Mendeley is maybe a bit easier to start with, but that could be partly because you can customize it less. However, for me Mendeley eventually did not work because it lacked functions that I definitely needed. If you browse through the discussion boards on the Mendeley website, you will see that there are quite some unanswered feature requests.
Zotero is an open-source program that has been around for a while, so there are many plugins that people have developed to solve specific problems and there is a user board where you get quick answers to your questions. You have to look into it a bit to set it up to do exactly what you want (although there are to do's online, even on youtube), but then there are many great features. For example, you can get a plugin to extract the annotations from your pdfs so that you can search them and with a click on the annotation or comment, go to the respective section of the pdf. It can also monitor your download folder and help you rename and organize your files into your literature folder. All very helpful.
I am very outspoken about the advantages of Zotero because I feel that it is much better than its reputation, and knowing about this would have saved me some time trying out other reference managers..