[CentOS] dovecot vs Cyrus-Imapd

I'm just preparing to build a new mail server for use at an ISP handling over 10,000 mailboxes and several domains.

We're currently using Qmail, but we'd like to move away from that since we have now changed our inbound mail systems over to Postfix.

Postfix will be receiving the mail, and I'm trying to decide between Dovecot and Cyrus-Imapd for the POP & IMAP service.

Does anybody have any advice or opinions about which of these two packages would be better for a large installation. I'd prefer if the package could tie into MySQL for user authentication, and all mailboxes will be virtual in the sense that they won't have local system accounts.

Thanks for your help. Bob.