Free. Linux is Unix, for all practical purposes.
Well, close. Linux is a free Unix "work-alike" in most respects, but it is definitely NOT Unix. It was specifically written to NOT be Unix, but rather to function mostly like it.
More to the point, the main difference between Linux and Unix is the kernel -- the innermost part of the operating system. They are completely different, but those differences are hidden below the surface of the command and application programs.
Both Linux and Unix use the GNU suite of system commands, and those commands are nearly identical in the two OSes in all respects. Linux and Unix also add their own OS-specific commands to the GNU suite.
The good news is, real Unix is free also, just like Linux. FreeBSD, NetBSD, and others are wonderful operating systems, if you prefer the great stability and security of Unix, and are willing to put up with a limited availability of commercial application software.
In most cases of "I want Unix", Linux is a much better choice, because you get a virtually identical experience from the commandline, and a much richer experience on the desktop.