This. The Spanish Civil War had some importance in terms of experience picked up by Germans and Italians there, but compared to a sea change in Chinese government, that's really nothing.
Two different questions: If it is about the War itself, I would say Spain, bacause of the international brigades, it beeing a model for proxy wars in the 20th century, new war technology, cultural impacts like Picassos Guernica, etc...
I don't know of similar things regarding the chinese civil war.
From the results, the answer would of corse be China, due to the size of the country alone....