TIMESTAMP is four bytes vs eight bytes for DATETIME.
Timestamps are also lighter on the database and indexed faster.
The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in YYYY-MM-DD HH:MM:SS format. The supported range is 1000-01-01 00:00:00 to 9999-12-31 23:59:59. The TIMESTAMP data type has a range of 1970-01-01 00:00:01 UTC to 2038-01-09 03:14:07 UTC. It has varying properties, depending on the MySQL version and the SQL mode the server is running in.
DATETIME is constant while TIMESTAMP is affected by the time_zone setting.