Amethyst tries to bring the XMonad experience to OSX. I think it does an admirable job, but there are some distinctions. Amethyst is simpler to set up, and is more forgiving to newcomers. It has a GUI for configuration, and an easily accessible list of commands. It also works on top of OSX's WM, so it's not so enormous a departure, especially compared to XMonad's fairly extreme dismissal of the mouse.
On the down side, XMonad really outshines Amethyst when it comes to performance. Amethyst is downright sluggish, where I've always found XMonad to be very responsive. Still, it's overall a true enough translation, and the sluggishness rarely actually hinders productivity. Overally, I think Amethyst is a capable daily driver, and a great intro to tiled window managers.