Well, I've switched between the two portal giants 3 times now, and can still not decide which one's "better". Technically, thy're both pretty much the same, but the main argument I've heard that "proves" postnuke is better is it's large development community. Unfortunately though, I can't seem to find an existing "large" postnuke community that
does anything - the develpoment side of things seems at snail pace :-/.
However, I do admit that I use postnuke at my site @muslimteens.net. I used it originally because I found how easy applying mods, and also getting mods was in comparison with phpnuke.
Recently though, I went to the phpnuke site, and development is fast - mods are better and more available, and they have started to use phpbb2 as a default forum in the new beta. Yes, we all know that SE would be better - but hey PHPBB aint bad

.
Currently, my opinion is that postnuke is better in the modding sense - as it has huge scripts such as Gallery and Postcalendar behind it. BUT, phpnuke's development is a lot quicker, and much more fluid in the reasonining of embracing new ideas quicker, whereas postnuke is slow and thinks about things too much.
What's your opinion?