As you'd expect, the answer lies in FaceTime preferences on the Mac, and in Settings on the iPad. First, take a look at the image at the top of this post. FaceTime settings are accessed by launching FaceTime, then selecting Preferences from the FaceTime menu (or pressing Command-Comma). In the preferences panel that appears, uncheck the iPhone Cellular Calls checkbox. Exit FaceTime and you'll never be bothered with the sound of your Mac "ringing" again.
Now, let's move to the iPad (image directly above). Here you're going to launch Settings, scroll down to the FaceTime button, and then toggle the iPhone Cellular Calls switch to "off" (white). That's it -- you're done. Now when a call comes in on your iPhone, you won't be bothered with the chorus of devices all ringing and you can just reach for your iPhone to answer it.