Apple’s Universal Dock is anything but universal. It won’t work with any iPads, it won’t work with the iPhone 5, and it sure as hell won’t fit a Motorola Xoom. And unless you’re using it two-handed, and you’re wide awake, it doesn’t even work very well with the iPhone or iPod Touch.
The Complete Dock, on the other hand, works with almost anything. Hell, if you wanted to dock your cruise ship in it you probably could.