One of the advantages to Cyrus is that it does not require that the mail users be related in any way to Unix users. A little bit of reading about Courier implies that that is not the case for it, but I could be wrong. Cyrus will likely be more difficult to set up, but you won't have to have Unix users for any of the Cyrus users. On the other hand, if you've got Unix users for all of your mail users anyway....