I am using courier as option for pop3/imap deamon in my ispconfig panel and in mailfilter syntex as mail drop without any real problem except very slow load of page in roundcube where there are more than 3k mails in the inbox. I just for the sake of curiosity I wanted to know as to which one is better and can I change it now without any adverse affect? also I wanted to use the spam filter sieve plugin for round-cube by using the option sieve.
Some poeple prefer courier, others prefer dovecot. There is no real benefit of one over the other. Courier is the older software, which means that there arent much new features added anymore. But this means also that it is very mature. I dont think that the slowness in roundcube is lated to the use of courier. Most likely its just slow because roundcube needs to process the list of mails. No. The mailbox layout is different. You would have to change the mail folder layout manually for every mailbox when you want to switch to dovecot. Sieve is a filtering language and it is used by dovecot. Courier has a different filter language named maildrop. The maildrop filter language has more functions then sieve.
The mailbox layout is the same (Maildir++), you just want to make sure that the index files are created for dovecot and that the imap message keywords are conserved. There's a script provided for that but make sure to test it on one email account before generalizing it to all you mailboxes. If something goes wrong then e-mail clients who use POP for example would have to download all the emails instead of just the new ones.
