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