Linux or FreeBSD, either way it's a dialect of Unix. And Unix really is the world's operating system.
When you run Linux, you'll be running KDE or Gnome to have a GUI.
When you run FreeBSD on Apple hardware, it comes with a really nifty GUI, called OS X (and cool aps, too).
(I've got a guild loyalty to FreeBSD, since it was written by another category theorist--my branch of mathematics, though I'm happy with Linux, too, since Torvald started with Minux--also written by a categorist. My household is standardized on Macs and my office desktop runs Linux.)
And, no, I don't buy Apple hardware out of any loyalty to Apple hardware, I buy Apple hardware because as of now, that's what Mac OS X runs on.
Most Linux stuff (well, a lot anyway) will run under X11 on OS X anyway.