IMO you can opt for the first one for the following reasons : #if you think that you don’t want backup, then you can use that internal HDD in your system #if you think that 1TB is not sufficient fot backup, then you can easily get a TB or even 2TB and just connect it to the casing #if you get an additional HDD which you have to connect to your system and you are not interested in opening your cabby, then you can just connect it via the casing.
And the pros of the 2nd one #if you think that this is final and you won’t need and some space for backup in future #if you have a budget constriction