XOOPS loads SO MUCH Faster and is way less of a server/SQL drain. I ran PHP Nuke on the same machine I now run XOOPS on and there is no comparison.
Also, the Admin/backend section of PHPNuke is od awful.. nothing like the simplistic module based XOOPS which is so much easier to get setup.
Found lots of little things that don't quite work with PHPNuke,, and my XOOPS site worked from the fist install without one single glitch.
Check out my band and XOOPS Site:
Dirrty Hairy - A Tribute To Hair Metal