My understanding how this all came about.
PHP-Nuke was first out, then some FB (php-Nuke writter) did not want to implement some requests and this postNuke came about. I'm not sure how Xoops came about, but NSN Nuke is a spin off of PHP-Nuke. Basically, PHP-Nuke is the foundation of all these.
I have heard a lot of people moving away from PHP-Nuke becuase FB has begun nickel and diming the community to death! After v6.5 you have to buy a membership to the "Club" to download upto v6.9. Now, I have also heard that v7.0 will be free to the community, but all the .x releases will fall under the same pay as you go concept.
All the other Nukes that are out there are currently free and I have not heard any rumbleings about them charging.
As for the Forum part, I believe that phpBB 2.0.6 is included within php-Nuke 6.9 (if not it's 2.0.5) There is a port for postNuke to use phpBB, but I am not sure what version is currently ported..
As for development - the core development of PHPNuke is by FB only, I'm sure he has helpers, but is kept behind closed doors. postNuke is developed in the open.
Both systems offer the ability for modules to be added and blocks to be added. This allows some customization along with a TON of skins to change the look.
Hope that helps, that's my current level of expierence with them.