I don't believe there is any functional difference,
except in where you go to re-activate it.

It is mostly conceptual or if you prefer related to
how you arranged it:  disabling is about taking
a positive immediate action, and expiration is
about setting up a timer to do it automatically at
some time in the future.

It is almost certainly different attributes on the
account properties (internally to the accounts
database) since there is more information to keep
for expiration.