IMAP on 993/SSL or 143/STARTTLS?

[Alex]

Hi,

I have a postfix+dovecot-2.2.13 system and have configured it to support
IMAPS on 993 with SSL/TLS. I'm noticing with users using Thunderbird, the
autodetect defaults to IMAPS on 143 with STARTTLS.

Which is preferred? Which is more secure? Which is more common?

Why would someone choose one over the other?

Can I ask the same question about SMTP and submission? Why would one choose
587 with STARTTLS versus 465 with SSL/TLS?

Thanks,
Alex