I prefer Color OS over MIUI and with the upcoming Realme UI it will be even better. In MIUI you can't even disable apps that come pre installed and which you don't use, in color os you can disable them. MIUI is riddled with ads in all system components and you have to individually turn off recommendation for each app, Color OS will introduce ads but will give a single toggle to turn them off system wide.
On the hardware front both are neck to neck so decide based on the OS and UI.