In my opinion, it depends on the education path you're choosing for your child. Are you planning to continue with the International School system, and even if you are, how much Chinese do you expect your child to learn? (Even with International schools the difference in the level of Chinese can be huge).
The exposure of the Chinese language would probably be greater in Yew Chung. I'm not sure about the Sheung Wan campus, but my 1.5 y.o. son is enrolled in DMS-Discovery Bay, and many of his classmates are non-Chinese. Personally i do not expect my son'd be able to read and write Chinese solely based on his DMS exposure.
Both Yew Chung and the Montessori system emphasize self-reliance and learning through all senses, which i think are very sensible ways to teach young children. Yew Chung would have more moral teachings whereas DMS seemed to be more academic.
Are you expecting your child to continue primary and secondary education in Yew Chung, if so, i think Yew Chung have a wonderful Kindy and you should stick to it. He/She would learn much Chinese as well. If you wish to apply for other International Schools, especially the ones that are more English based, then DMS would be a better option.