There are lots and lots of differences. Keep in mind that China has at least 8 different cuisines so while the foods in northern coastal China have similarities such as the use of wasabi and lots of seafood, in the western part they use mostly lamb and eat noodles. Its hard to do a true comparison but I’ll try. – Raw vs cooked – Traditionally Chinese distinguished themselves from other cultures by saying they had mastered fire, and thus had quite a taboo on raw food for most of Chinese history.
Japanese clearly like raw foods. – Role of visual aesthetic – While both cultures care about color and appearance, Japanese will consider a dish good sometimes almost solely based on how visually pleasing it is. – Rice – Japanese rice is shorter grain and stickier. Northern Chinese rice is also short grain, but in general Chinese use longer less sticky grains. – Spice – China has some very spicy cuisines such as Sichuan and Hunan. There are few very spicy dishes in Japan.
– Influences – China was a huge part of Silk Road and had lots of influences from other countries.
There is still a lot of Muslim influence in many parts of Chinese cooking. Japan has incorporated more modern gourmet food from the West in recent history. – Tea – Japanese green tea tends to be steamed while Chinese green is often roasted in the production process so they have very different flavors sometimes. – Alcohol – Japanese sake tends to be less strong then Chinese rice wines. Chinese rice wines fill up the whole room with a strong odor and are as high as 90 proof.