Sorry to the guy who's box got hacked via "DNS". If you were running an exploitable version of Bind on an *bsd box you would have been nailed just as well.
Given a competent admin of either OS, I believe they are equally secure. I can't even remember the last kernel-level type bug that would allow remote access to either of the system. It's all in what apps you run.
Granted, some distributions are better off OUT OF BOX than others, but when you get down to it, no one should be running anything they want to serve critical stuff out of box. If you do, you deserve to get nailed.
I don't know how many times I've seen people with dedicated boxes just leave what is installed on there by default alone (i.e. EVERYTHING it seems. Bind, sendmail, telnetd, some form of ftpd, apache, etc. etc.) and NEVER upgrade it. Much less actually *gasp* watch the security alerts for their stuff.
This is one reason I'm more of a shared hosting or managed dedicated (where your host admins it for you) kind of guy. Let those who know what the hell they are doing admin, and guarantee security.
And if someone else is managing it, why the hell do you care what underlying OS it is? You will not see a difference between Linux and *BSD in terms of performance (assuming competent admins again), so why care? I'd rather be using what my admins are most comfortable with than using something for religious/FUD purposes.
In both OS'es, it comes down to who admins the box. Period. A well administrated network will not be getting hit with your script-kiddie "DNS" (Bind) attacks, as they will have upgraded all the machines the second the exploit hits (we hope, right?).
So for myself since I do hosting, I highly prefer Debian. Why? Just because I know it, can easily do common tasks, and can easily automate my own apache packages and such so I can upgrade 15 servers in the time it takes to compile a new package on the local debian mirror.
However, someone else may be able to make freebsd hum, and if I were hosting with them I'd definitely want them to be using it.
anyways, this got long.
peace,
-Phil