There's a reason why "sub" can be part of "domain", and means "it is the same place, just a bit farther down the stairway..."
There are technical reasons for subdomains and there are reasons for subdirectories. Examine the need the sub-divide and move from there.
Me? I dilute as little as possible as that DOMAIN is the reason and that's what I protect, and sub-dividing that is like cutting my ranch into timeshares.