>>4
what? neither are certified POSIX compliant. The closest thing to either that is is Mac OS X, which is certified against the Single UNIX Specification
>>2
Linux is certainly not UNIX-compatible. It's a unix clone.
>>1
In my opinion, the real difference is between FreeBSD and Linux distributions. Most Linux distributions are a set of discretely packaged components that come together to form a full system (using package build systems and patches to build packages using source from upstream revision control repositories or releases), whereas FreeBSD is built from a single revision control repository.