China is the world's largest economy based on PPP.

Statistics from world bank

http://databank.worldbank.org/data/download/GDP_PPP.pdf

China is expected to overtake USA's economy in nominal GDP by 2030.

http://statisticstimes.com/economy/projected-world-gdp-ranking.php

China's economy will be 50% bigger than USA in the year 2050 (see below).

Statistics from PWC

http://www.pwc.com/gx/en/issues/the-economy/assets/world-in-2050-february-2015.pdf

Whose side do you want to be on? USA or China?