The primary difference between China & U.S. defense spending: U.S. R&D is paid out to researchers and engineers who design new technologies and build the production facilities. China R&D is paid out to computer experts who find ways to steal U.S. R&D, a much cheaper route to obtaining the same end result.
Well that and our military is vastly more powerful and capable than China's. We can project power almost anywhere in the world on very short notice and for a very long time. China has little ability to do this beyond the territory immediately surrounding their country.
People often forget the US's greatest advantage (or at least one of our greatest advantages), our unparalleled logistical capabilities. It might not be as sexy as tanks and fighters but it's what actually lets us do what we do. It's also very expensive.