User numbers.
If the organization has Exchange, but does not want long-term coexistence, the final deciding factor is the number of users.
If there are fewer than 2,000 users, the recommendation is an Exchange cutover migration.
if there are 2,000 or more mailboxes to migrate, then you need to review the version of Exchange Server again.
If it is Exchange2003 or 2007, then staged migration is the answer.
If the organization is using Exchange2010 or Exchange2013, then you need to implement a temporary hybrid Exchange arrangement while mailboxes are migrated.