don't know postnuke...
have to look into that.... for what i've seen... it's just a modified version of phpnuke...
meaning:
both version need a LOT of tweaking before it does what you want it to do... but when you do.... OH MAMMA!!!
maybe postnuke is getting more attention now (
www.nukeaddons.com), but i'll stick to 'my' phpNuke till postnuke *really* becomes better.....(!)