Update-manager is a frontend for the apt package management system. It allows you to perform some actions of the command line tool apt-get in a graphical interface. Developed by Michael Vogt
License: Open Source
Synaptic is a GTK+-based graphical user interface for Advanced Packaging Tool (APT)—the Package management system used by Debian and its derivatives. Synaptic is usually used on systems based on deb packages but can also be used on systems based on RPM packages. Developed by Daniel Paarmann
License: Open Source
In summary, Synaptic provides a comprehensive package management experience with support for PPAs and advanced features for managing software, making it ideal for users who want greater control over their system. In contrast, Ubuntu Update Manager is streamlined for handling system updates and security patches, focusing on ease of use for the average user.