In 1978 (the time of Mao's death), the GDP per capita would have been most divergent, owing to the divergent paths taken by Japan and China. For Japan there was the Meiji Restoration, and heavy investment by the USA post WW2, while China got a civil war followed by Mao's not so good economic policy.
But I'm wondering if Japan's economy was in better shape even before the Meiji's Restoration?
I would say it'd be very clearly Japan, but almost entirely because of the Taiping Rebellion.
From 1850-1864, one of the largest civil wars ever was fought in China between the "Heavenly Kingdom" and the Qing Dynasty. The war was notable for being one of the first instances of "total war," with civilians essentially being dragooned into the fighting, both sides attempting to deny territory by burning agricultural lands, the destruction of some 600 cities, and upwards of 20 million casualties. China did not recover from the war until probably 1870 at the earliest.