Keeping session info in $_SESSION or in database?

Hi all, This is THE question that bothers me for a while... I always was keeping session info, like user ID, organization ID, selected book ID... within $_SESSION array. Main reason is to access and maintain it faster than keeping them inside session table. And, also, one less mysql connection. Though, in last project the $_SESSION grow up to around 30, even 50 elements of the array. And several people mentioned it's better to keep so big session data in mysql than in $_SESSION. My question is pros and cons $_SESSION vs. mysql session. And, if the amount of data is only reason, when is better to keep all data in $_SESSION and when to store them in mysql? Thanks for any help, LAMP