Overseas Chinese control, by my estimate, $2 trillion of liquid assets that can be mobilized instantly to punt on anything from European football games to Shanghai property. This source of money is both the boon and the bane of China's economic development.
The two city economies became rich by being the gateways to the rest of the world for their large neighbors. However, globalization has allowed their neighbors to bypass them and access the global stage directly. Both are trying to get back into the game, but in different ways.
China is a developing country that saves too much, and the United States is a developed country that spends too much. The result is a big trade gap. Does something have to give? Yes. But it's not clear when that will happen, and in the meantime both economies are performing pretty well.