Video: Alfresco vs Documentum

Alfresco is a Free enterprise content management system for Microsoft Windows and Unix-like operating systems. Alfresco includes a content repository, an out-of-the-box web portal framework for managing and using standard portal content, a CIFS interface that provides file system compatibility on Microsoft Windows and Unix-like operating systems, a web content management system capable of virtualizing webapps and static sites via Apache Tomcat, Lucene indexing, and Activiti workflow. The Alfresco system is developed using Java technology. Alfresco open-source alternatives are Nuxeo, Liferay Portal.
Documentum provides management capabilities for all types of content including business documents, photos, video, medical images, e-mail, Web pages, fixed content, XML-tagged documents, etc. The core of Documentum is a repository in which the content is stored securely under compliance rules. This repository appears as a unified environment, although content may reside on multiple servers and physical storage devices within a distributed environment. Free open-source Documentum alternatives - Alfresco, Nuxeo, Liferay Portal.
Face to face in the news:

2016 - Alfresco integrates its ECM and BPM. Documentum is in panic

Alfresco announced a tight integration between its enterprise content management (ECM) platform, Alfresco One, and its Business Process Management (BPM) platform, Activiti. It's the climax to years of effort to pull the two platforms closer together, and rounds out the functionality of both to manage both business content and processes. Today’s announcement enables Alfresco to compete in Process and Collaboration Management (PCM) market. Alfresco is bringing to that market offers enterprises a broad range of content management and content driven processes. In effect, it combines BPM, ECM and records management, as well as offering integrations with capture and e-signature vendors.

2008 - Documentum 6.5 - ballance between Enterprise 2.0 and IT compliance to win over Alfresco

Documentum 6.5

EMC calls the new Documentum 6.5 the "perfect balance: between the modern Web 2.0 technologies and compliance with enterprise security". They say "You can provide the world's best enterprise content management system, but if end users do not like to work with it, the vast amount of information just does not get into the system. Documentum was always considered the best ECM-platform, but when it comes to user interface - it wasn't so good". The new version of Documentum includes 3 new user interfaces: Centerstage Essentials (web-interface for knowledge workers that allows to create online workspaces with social tools such as microblogs, forums, tags, ratings, galleries), Media WorkSpace (Flex-based interface for managing media content, that also provides social tools) and MyDocumentum (a simple desktop client intended to access the most commonly used content, even offline).