Linux != UNIX -- Linux is a (mostly) complete rewrite to mimic minix with no ties to real UNIX. Linux itself is just a kernel with the GNU utilities used for userland.
*BSD == UNIX -- BSD is UNIX, but isn't allowed to be called "UNIX" due to stupid copyright issues ... The BSD's all have a common history dating ~14 years before the first line of the Linux kernel was written.
Generally speaking, UNIX (BSD, AIX, Solaris, etc) are more "industrial strength", stable, etc. than Linux. Linux fanboys will argue this point, but it's mostly due to their not knowing any better.
/me puts on the asbestos suit