Which is the best practice: Saving game currency in Game Instance as a variable or save it with Save game function wich saves it as FileName.sav file?
It’s not a question of best practice, it’s a fact about how you want it to work.
If you don’t care about the money after the player closes the game, then game instance.
If you need the player to have the same money when they re-start the game as when they quit, then you have to use save game.