CMSs are great things for certain types of sites what do you plan on using this for? They are great for Clan sites some businesses (not web hosting at least not imo) I like XOOPs personaly but that is me. I use it on a couple of the sites I own/operate not to mention fail to keep up to date.
Pro: Easy to use
Pro: Not as many errors
con: not as powerful as Nuke
con: you almost HAVE to know PHP
con: Lots of errors
Pro: ez to port things into (if your a programmer)
Xoops is a Nuke also. Actually, PostNuke AND Xoops were forks off of the original PHPNuke.
They are all setup similarly yet differently and things in common as well as thing different. It basically depends on the server configuration and what the site would be used for as well as the level of the user.
Some things I noticed when I was setting these up is that PostNuke installed fine on all the servers I tried it on whereas PHPNuke only installed on a few, but once installed, PHPNuke had more features and was easier to understand and use. But PostNuke is supposed to be better written to run more efficiently. Xoops required more file permission changes and wasn't totally database driven like the other 2 so it required some special CHMODing. It was also lacking in some of the features the others had.
Overall, It depends on several factors as to which to use. A little bit of research by doing a search on them here will reveal some answers.
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.