You didn't really tell us why you want to use unix, because i believe unix caters to a much more specific niche than linux does, and technically, linux conforms to POSIX standard, that makes it a unix-like system, which is quite close to the real thing unless you want to get lower than user-level tools..
Unix is just a trademark, technically, freebsd is not a unix in the sense that you cannot call it as such.. Since it didn't bother to get certification from the unix foundation..