A portal is generaly a script for fetching and displaying content.
A CMS is a content managent system, meaning that it can serv and host multiple types of content, and should allow you to manage this content.
Deciding what is right to for your site depends on the site itself. First of all you need to figure out if a full blown CMS is what you need, then if you decide to use a CMS you will have to pick the one that is right for you.
Choosing a CMS can be hard, many produce way to much overhead, others will simply not be suited for your server/site.
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