wordpress is great for those that don't like to muck around, don't need full control of ads, user permissions, don't know php etc. postnuke is the one with the best security and best control of everything. wordpress also has too many updates - so if you have lots of modifications that can be a pain with each new upgrade.
postnuke is about to be rebranded and released to world, which will take notice - and you heard it here first. The last release has been stable and secure since nov 2006! How many cms packages can claim that? Set it and forget it baby!
Some examples:
http://das.utah.gov/main/ http://csu.qc.ca/ http://www.nymf.org/
I use them both - wordpress is best for quick and dirty websites, blogs and article/news websites. postnuke is much better for full-service websites, but is not for first time users to a cms as it can overwhelm new users.