These are conceptually less of drivers than actual filesystems that you mount under /var/lib/docker and each comes with its own set of pros and cons. Generally, they both have performance impacts as opposed to some of the other options and have a high memory overhead but may provide you with easier management tooling and/or higher density storage. Since these drivers currently have marginal support and I have heard of many critical bugs still affecting them, I would not advise using them in production unless you have very good reasons to do so. If the system has the appropriate drive mounted at /var/lib/docker and the related kernel modules are available, Docker will pick these next after aufs.

Note that the order of preference here doesn't mean that these two storage drivers are more desirable than the other ones mentioned in this section but purely that if the drive is mounted with the appropriate (and uncommon) filesystem is at the expected Docker location, Docker will assume that this is the configuration that the user wanted.