I think that America still has the upper hand over both though.

Obviously it is possible that China leverages it's holdings, however, America can just default on foreign debt if they choose to do so. The Chinese military power does not match America's and thusly they cannot collect the outstanding debt by force.

Additionally, the US or the FED could basically devalue the debt that China holds by printing more and more money. Realistically, China's assets would soak up 1/3 of the loss in value of the dollar without hurting the American economy at all.

Obviously, either would be a drastic measure of the American government, however it shows you that America is far from done or losing the upper hand.

In the case of money printing for example, I think the Chinese economy would be hurt more drastically then the American, because the additional dollars created would be spend withing the US and not in China practically punishing China's economy if the US government implements a sensible policy that supports the American population via economic growth.