Filesys::DiskFree calls and parses 'df' rather than use libc functions like statvfs. I see this as likely to involve more overhead. The API does not look that nice to use; there is a separate initialization step that must be called after new() - this could be called automatically on the first use of an accessor, for example.
The documentation is pretty good, although it would be nice to have the copyright/license in the pod rather than having to read the source code.
All in all, I would choose Filesys::Df over this module.