US has risen to become world's top creditor nation since World War One, maintains throughout World War Two, until Reagan times when position moved in opposite direction, and nowadays world's largest debtor nation. Yes, Japan replaced US as world's top creditor nation since 1990 and maintains the position till nowadays.
The net position is mostly from
FDI, Japan owned overseas FDI assets as high as Yen 150.365 trillion (end March 2016) while inbound FDI liabilities is only Yen 25.591 trillion, net FDI position is Yen 124.774 trillion. Similarly in
Portfolio Investment. Japan owned these net assets, but associated economic activities (i.e. GDP) data doesn't account in domestic data inside Japan. In plain language, Japan owned a lot of overseas assets and conduct economic activities outside home soil, that's why home data looks "stagnant".
Though China Mainland is also a creditor nation, but the makeup of asset is entirely different. At peak time China Mainland holds
60% of total foreign assets in FXR (largely US T-bills to support fast growing US public debt), but less than 20% in FDI. In comparison Japan holds only 14% of total foreign asset in FXR, Germany only 2%. China began a strategic shift in 2013, pivot away from FXR and towards outbound FDI (Africa, OBOR, LatAm, CEE). Now China also wants to swap the debts for real assets in US, but so far this path isn't too smooth due to US "national security" concern, which even tries to block Chinese M&A in other countries.
(P.S.: International practice accounts China Mainland as an entity, so strictly speaking, nowadays China should be top creditor nation since Hong Kong and Taiwan, both are also among top creditor economies, should be included.)
On topic, given the sizes naturally China and US "grow" most, aka in GDP, but the way of "growth" is entirely different if not exactly opposite, there is no comparison between the two. China does not issue international reserve currency, doesn't have unlimited international purchasing power nor unlimited international debt ceiling. US consumption and public expenditure (military spending will grow faster than others) will continue to drive US growth, in the contrary China growth will continue to be driven by domestic savings (hence domestic investment), industrial-production, exports and outbound investment (acquiring overseas assets or greenfield).
https://defence.pk/pdf/threads/who-are-worlds-top-10-largest-creditor-nations.455610/
http://www.mof.go.jp/international_policy/reference/iip/201609a.pdf