An important difference is that Japan is a considerably smaller country, and to surpass the US they'd have to pull off something really remarkable and raise their per-capita GDP 2-3x above that of the US to surpass it absolutely.
China is more interesting and realistic because all they have to do is achieve a per-capita GDP somewhat similar to the US, and their vastly larger population makes their economy (and power) much bigger too. Japan didn't eat the world, but they did achieve a per-capita GDP about 2/3rds that of the US. If China just gets that far it'll already change the game beyond imagining.
China is more interesting and realistic because all they have to do is achieve a per-capita GDP somewhat similar to the US, and their vastly larger population makes their economy (and power) much bigger too. Japan didn't eat the world, but they did achieve a per-capita GDP about 2/3rds that of the US. If China just gets that far it'll already change the game beyond imagining.