I only want to use it as external storage attached to my Mac - should I format the disks in ExFat or HTS ?
I assume you are saying “APFS” because its macOS?
The only reason to use exFAT is for cross-platform compatibility.
Otherwise when you 100% sure the external drive is for macOS only, format it to APFS is an good option because it provides OS-level snapshot, encryption … etc.