It probably depends on how you look at it, but I don't believe the MACH kernel is unix based. The userland is though, as it is taken from *BSD (primarily FreeBSD, but Open and Net also show up on my system).
There are plenty of differences. The price. The hardware availability (even if you choose just Darwin). The userland is BSD based, so that can be a difference in the minds of some people. Authentication is handled differently. Hell, most of the system that most people look at is different. Getting used to the quirks in OS X can be a pain (linux has its own quirks, as does every other OS not mentioned in this thread thus far).
There are philisophical differences that can mean something to some people. OS X is an operating system and is developed as such. Linux isn't. Neither way is right (sorry), and each has its advantages and disadvantages.
To sum it up in one over simplified sentence: "OS X and Linux are different operating systems." There
