I would first think about your user requirements. If you find that Alfresco is a good fit and does the job well then there really is no need to look at Liferay or anything else.
FYI - Liferay is targetted at sharing information between communities. It provides about 60 portlets out of the box but, as it supports JSR-168 and JSR-286, you can easily plug-in 3rd party portlets (e.g. integration with ECM systems, ERP systems, etc). Very basic document management is provided via the embedded JackRabbit repository. Web publishing is supported through Liferay's Journal CMS portlets. Alfresco can be integrated to provide more comprehensive DM and WCM.
It sounds like the question you're really asking is what's the difference between an ECM and a portal. I would suggest that a portal is a one stop shop for all corporate information (e.g. search, user directory, announcements, forums, integration with external systems, etc) whereas an ECM provides functionality to collaborate on and manage content. The lines seem to be blurring though. Alfresco supports dashlets, forums etc which are portal-like features.
