I'm looking for a posting script, like phpnuke or postnuke. Which of these is more secure? Are there any others out there that are more secure? I know phpnuke and postnuke have had exploits in the past. Just curious as to how secure they were.

A posting script? I don't think Post Nuke or PHP Nuke are posting scripts.

If you need a posting scripts in PHP try some of these ones - they only post so you can simply include them or put a header and footer to suit your site.
http://www.hotscripts.com/PHP/Scripts_and_Programs/Guestbooks/index.html