Doctrine: ODM vs. ORM

Question

I am considering using ODM / ORM / mix ODM and ORM in my new Doctrine 2.0 project. When should I use ORM (MySQL InnoDB) and when is ODM (MongoDB)? Should I mix it in one project?

The pseudo structure of my application (e.g. Google Reader):

• user {id, login, password, email}
• channel {id, url}
• entry {id, channelId, name, body, date}
• entryValue {entryId, userId, value}
• userFriend {user1Id, user2Id}

Most popular features:

• getValuesOfNewsByUsers (newsId, [userIdArray])
• getEntriesByUserWithMinValue (userId, minValue)
• areFriends (user1, user2)
• getEntryValues ​​(userId, [entryIdArray])

I have to be prepared for billions of rows in the entry and entryValue tables.