This is fairly simple. Open a Terminal type:
defaults write org.gpgtools.gpgmail DefaultSecurityMethod -int 2
Close Mail, open it again and you are set.
No matter what you prefer either GPG/PGP encryption over S/MIME – it is always a good thing to encrypt your email.
Setting up S/Mime on a Mac is simpler then you think.
Go for example to Comodo, get a free certificate by filling out the simple form (name + email), wait for the email, click on the link, download the certificate, double click, restart Mail – DONE!