Ranged vs. Siege seems like it could follow the Iron units vs. Horse units model. If there were no promotions to differentiate Iron units from Horse units you would only build Horse units (assuming you have the resources to choose from); 4 move beats 2 move every time. But Iron have a city attack and defensive bonuses and while Horse have a city attack penalty and no defense bonus, differentiating the two unit types neatly and cleanly. Ranged already have a penalty when attacking ships (going off memory here, apologies if I'm mistaken). Give Siege a small penalty when attacking land units and the two unit lines will each have a clearly defined purpose while remaining versatile.