Its a personal thing, no one answer suits everyone.
I'm suspicious of any CD RW method because it seems this is the least reliable of storage media, though you are using 3 disks in rotation and would presumably have a warning if a disk became unreadable.
For me it is extra HDs, both internal and external.- Over the top for some.
A pen drive is so useful anyway, but strikes me as vulnerable (to loss) and fairly expensive for backup purposes.
But I said - its personal