Installing OpenBSD is very easy, just donīt ask about dual booting because then the hard part comes
I am not an expert on this matter to say that OpenBSD is more secure then FreeBSD, but if you are not a newbie then you would know how to close services you donīt need and secure a FreeBSD box. In OpenBSD everything is closed, you canīt even telnet to the box. SOme people might prefer this as a start.