It's entirely dependent on the design itself, and isn't restricted to whether the mount is on a quad or biped. The intent was to cover all the different types of turret-like systems we've seen in the game before they actually invented a proper turret system. Give the 2-point version to anything with a less than 360-degree field of fire (like the original Unseen Grand Crusader), and the 3 point-version to anything with an unrestricted turret (like the Unseen Goliath). It is possible to design a quad with a restricted turret setup, though I can't think of a canon one off the top of my head.
Does that clarify the intent?