i prefer open source os because,
1.as the open source users are less compared to closed source os users, hackers usually target the os where large number of users are lresent.
2.in closed source os the full admin access are given to the users so the will be vulnerable to malicious mails because some time amateurs will respond to spam mails unknowingly, where as in open source os the users must get authorization right to execute the mail.
example: linux- open source os & microsoft windows- closed source os.
here both the os face the threat but everything comes down to the user...like how well he can manage the os