diem

Which symfony-based CMS is the most extensible?

Hello, I'm looking for a symfony-based CMS, and I have heard about Diem and Apostrophe, but none of them offers all of the features I am searching for. In your opinion, which of these CMS' is the most easy to extend? Do you know any other Symfony-based CMS I would want to consider? EDIT : By 'to extend' I mean adding probably unsupport...