No one can make citation management system decisions for you. However, here are some questions you should ask yourself when deciding among them.

1. Do you switch between computers often? If so, a citation management system that syncs to the cloud might be the best choice for you, as you would be able to "pull down" your citations to each computer.
2. Do you use multiple operating systems? Some citation management systems only support one operating system. Other solutions (i.e., Mendeley and Zotero) support Windows, Mac, and Linux. They're better choices if you see yourself doing research on all platforms.
3. What do people in your discipline or research group use? Unless everyone is just saving things to a BibTeX database (you can export to that format from citation management systems to share and use citations, so that's a non-issue), you will get more value out of a system if you are using something in common with people you collaborate with. Ask around and see if others have preferences. Of course, you will have to weigh this against #2: If your associates use Papers for Mac and you switch between Mac/Linux often, that is not the best choice for you.
4. Other than citation management, what does the software do? Some people prefer all-in-one tools, and others just want to have a program manage their citations. It's really up to you. Mendeley, however, lets you do several things that many other pieces of software might not: (1) all-in-one note-taking and annotation functionality for PDFs; (2) linking a citation to multiple documents in the case of supplemental data sets, methods, and other information related to the paper; and (3) engaging with a community of scholars via a social hub that supports a CV-style user profile.

That said, here is a guide Mendeley has created that compares its software to other citation management platforms. Again, think about which features are important to you and which ones aren't as you look through it: http://www.mendeley.com/compare-mendeley/.

It's important to note that Mendeley uses the freemium service model: You have free account access up to a certain amount of space (currently 2 GB), and there are limitations on the number of restricted groups you may have. That said, for most researchers' purposes, this initial amount of space is fine.

If you want to get started with Mendeley, the following links will walk you through account creation and the Mendeley software download.

Kayleigh Bohémier
203.432.9519
kayleigh.bohemier@yale.edu
http://guides.library.yale.edu/prf.php?account_id=54485