I have not worked with portals for quite a while, but I guess the answer is : It really depends. :)
Seriously, in your requirements context I would compare Alfresco Share with Liferay. Both compose pages in a similiar fashion with Webscripts/Portlets components and have security baked in.
I think the most important factor to consider is your skill set.
If you feel at home with Java, the Portlet spec and server side logic : Lean towards Liferay. Alfresco Share/Spring Surf should serve you better if you prefer Javascript in the browser and JSON/REST style programming.
Besides, validate how strong your document management requirements are.
Do you really need super sophisticated search, workflow, versioning, security and the like ?
Sophistication increases complexity. That is also why I would be careful before introducing them both at the same time.
Another point to have in mind may be documentation - guess this point goes to Liferay.