Anyone saying China is overestimating China's capabilities and underestimating the United States's capabilities. This would happen under normal circumstances (no Iraq, Afghanistan, etc.)
The United States military is much more advanced technologically than China which makes up for it's lack of manpower compared to China.
While the PLA is much larger than the United States Army, the United States Navy and United States Air Force are much more advanced, larger, and more powerful than the PLAN and PLAAF.
The United States would have a large amount of allies to back up on if the Chinese attacked American soil (NATO, Australia, Japan, South Korea, India, etc.). China just basically has Russia and embarrisingly North Korea.
China has a few areas that have significant independence movements: Taiwan, Tibet, and Xianjing. The United States lacks that.
The United States would win, but it would be a very tough battle.