RUF is a car maker, they have their "own" line up. You could compare them better to Alpina and BMW. Brabus is just a tuner, who has been around for some time now. They were always known to build the fastest and madest Merc's
Well, there certainly is some sort of connection between Brabus and Mercedes-Benz. For instance, you can buy a Brabus Smart (forfour, fortwo, etc.), directly from Smart.