I would say the garden has more oxygen. oxygen and carbon dioxide is completely 2 different elements. and since there are more plants, most likely, if it has enough sunlight, the garden will have a better oxygen, because for plant photosynthesis it releases oxygen as their by product.
however please note that if the garden is dark, no light, the garden will lower the oxygen concentration because plants respire and they release carbon dioxide instead. photosynthesis only occurs in light condition as its the source of energy.