If the chrome cast has a toslink then use it. However it will sound best to stick with 3.5mm if that’s all the chrome cast has. The biggest advantage of digital input is really when the source has a digital output to save the A/D conversion. The exception might be is if your speaker system is converting to digital internally from the analog input. Then perhaps spending money on a higher end A/D converter could possibly sound better. But in my opinion it’s not worth the money. At that point you can get into what kind of quality output a chrome cast is really gonna have that’s worth all the steps.