On Pinnacle class ships, I recommend CO (especially if you are not 4 or 5*). The separate restaurant is not different in menu, except 1 different added entree each night) but the service and ambiance is much better than the large, noisy and sometimes chaotic MDR (IMO). You pay for CO when booking and choose your cabin (upgrade within same meta category unless you pay extra (may be discounted) for different category.
The CO restaurant is not a reservation type, just walk in (or tell the host on the first night what time you want and they will look for you, although they cannot reserve a particular table...wink wink)