Explain the difference between china and japan, in the 19th and 20th centuries.

Well Japan had a long relationship with the Dutch and it started in 1640s, they  also started learning from the Dutch since then.Christianity eventually lost the Qing Emperors' trust. In 1746, the Qian-long Emperor issued an edict which forbid the spread of Christianity in China, as a result many missionaries were sent back.The Dutch mostly focused on business,the Japanese were able  to learn western science without the tension of converting to another religion. 
As for China missionaries wanted to convert, and paid attention to improving the relationship and adapting the rituals,the Chinese tended to be more reluctant in accepting them and their science that they brought.
China was stronger in Confucianism, and didn't want to tolerant the foreigners. Japan, had a more open minded history which enabled them to be very malleable in this transition.