comparing form the health angle
I prefer Japanese food as most foods in Japan are not all that greasy and salty as far as I can contact. Chinese food is too spicy and salty to long-term taking, what is more, the safety of food cannot be expected in China.
Typo: "...contribute to this phenomenon."
It is always hard to generalise, because in every country there are healthy eaters and unhealthy eaters.
There is, however, general consensus up to a point on the basis of World Health Organisation Research and media reports. There are of course academic studies but am a mere layman and have not read any of those.
Here is what I think the international consensus is:
1. Both the US and China have an ever-worsening problem with adult, teenage and child obesity.
2. In both countries there is a trend for people to consume more and more junk food.
3. Food research and good food consciousness are more advanced in the US, but still a large part of the population are ignorant of such or do not heed such.
4. Food safety is better in the US.
5. The Japanese are on the whole some of the healthiest eaters in the world, following a low fat, low salt, low sugar and low glycemic index diet. Fresh fish, a near-religious insistence on fresh and safe produce, a worship of hygiene and health, and a people who are culturally disciplined all contribute this phenomenon.
6. Americans tend to eat too much meat, fat and sugar. Self-reward and pleasure in and through eating are much more evident in the US (than say in Japan).
7. The Chinese tend to eat too much salt, fat, sugar and carbohydrate.
8. In China Hong Kong, Cantonese food in may be one healthy exception, because of its heritage emphasis on fresh ingredients, steamed fish and vegetables and quick stir-fry technqiue using minimal oil, and also as a result of a better educated and more international-minded population more ready to accept the concept of a high-fibre, low fat, low sugar, low salt and low glycemic index diet.