Having used both Subversion and Mercurial, I think the best starting points so you can get your own conclusions about them are these posts:
Subversion: [Setting up and running Subversion and Tortoise SVN with Visual Studio and .NET]
Mercurial: [Hg Init: a Mercurial tutorial]
In short, Mercurial is a distributed version control system, while Subversion is a centralized one.
On Subversion, you “commit” and “update” directly to/from the central and unique repository.
On Mercurial, you “commit” and “update” to your local personal repository. Once you’re sure your code is ready, you can “push” your changes to the central repository. To get other people changes, you have to “pull” from the central repository.