The problem with the US is not democracy. It's a small power cabal which controls the economy, controls policies, and controls whichever party gets elected. It is capitalism at its purest, where capital is king and is beyond the remit of governments.
The trouble with a one-party govt like China's is that if the govt is dedicated to serving the people you have a long period of peace, development and prosperity. but if the govt turns rogue there's no peaceful means of changing the govt. Only recourse is to violent coups and revolutions. China's long dynastic history bears this out.
Two takeaways:
1. Good governance is key
2. Democracy (and for that matter all manner of autocracies) does not guarantee good governance, but it remains the only system that allows the people to change their govt peacefully.