Japanese 7,63 ammo was made for a small number of imported Subguns of the 1930s ( Solothurns and Bergmanns) and from 1937 onwards, for any captured M96 brooms and Chinese Subguns ( also Steyrs,Solothurns and Bergmans, Both European and Chinese made)
So in comparison with the 8mm Nambu, which was the Japanese Service Pistol and WW II Subgun round, the 7,63 made by Japan would be naturally rarer, also seeing that it was mostly used in China, and not in the wider Pacific Theatre ( although who knows exactly what was used where, except by diligent research and finding specimens?)
I would say that collectible, confirmed Japanese 7,63 is more valuable than 8mm Nambu, so you should be "in front".
Remember that some Chinese Plants ( whether under Japanese or Nationalist control,) also made 7,63 without headstamps...so a question still remains...Loose rounds may be difficult to ID definitely. An original full packet (Japanese Plant) would be helpful for cartridge comparison.