CMS system for a large content user contribution site

Would like your ideas on which cms to choose

[Kongen]

Hi

I need a cms system, for a new large content site (hopefully), and I would like your expert opinions on which cms system you think I should choose.

I have the following requirements

Very search engine friendly

Users can contribute with articles

I can download "themes" so I don't need to design it myself

articles can be indexed in categories

as "light" as possible (has experience with phpnuke, but find it too slow and big)

must be able to comment on articles

must be php/mysql

Must be free of charge

I am looking forward to hear your ideas :-)

[Kongen]

Ok, found cms matrix in another thread, and now im down to Drupal and Mambo.

Any notes on the difference between these two systems? Then i would love to hear about it.