Japanese Champs are really good. Another quick difference:
Elite Samurai costs 950f and 875g
To get to champs it costs: 1350f and 555g
Ultimately the gold savings of the champs themselves, and the gold savings on their upgrades (in addition to being trained from barracks) makes champs the obvious choice.
So maybe make samurai cost 60f 20g (10g cheaper), and reduce the gold cost of the elite research to 650g (225g cheaper).
With this change I’d see the main benefit for samurai being a) fighting UUs b) fast tech switch into melee in imp to clean enemy trash.