I've personally havent had any problems with phpnuke5.6...
However, since there are dramatic changes between the 5.x and the 6.x versions, I've had to hack mine to accept 6.x modules and such. I find that versions 6+ have tons of bloat, and a bunch of issues.
I've tried postnuke, but I keep going back to phpnuke, and I've seen that postnuke "upgrades" tend to make many modules no longer work without being edited.
I suggest you try both and see how you like them, and make the decision based on what you're comfortable with.