I have had both and neither one holds a candle to "Chess Opening Essentials." I promised myself I would never buy another opening book but wanted something that was pretty comprehensive and explained the concepts for all the major openings in an understandable manner.
I have the first two volumes and will be getting the last two shortly. They are the best reference by far in looking up openings and grasping what they are all about. MCO (full of errors) and NCO are not intended to elaborate on ideas behind the openings and should never be bought with the idea of learning opening theory by amateurs.
Which is the best for learning openings? MCO or NCO?