The index comes first
TOPIX covers a broad range of Japanese listed companies. The Nikkei 225 follows 225 selected companies. Because their rules differ, the two indexes can produce different returns even when the Japanese market moves in one direction.
Compare funds within each group
Start with the TOPIX ETF list or the Nikkei 225 ETF list. Then compare the trust fee, trading unit, and fund type. Some listings use leverage or inverse exposure, so check that the product matches the index exposure you intended.