Because JForum is a J2EE implementation of phpbb, moving to it will probably be less difficult for users (and maybe administrators as well). We have installed JForum (and Roller for blogging) on the same appserver with our XWiki instance. The integration of the three was limited to placing links to them in XWiki's viewheader menu. This has (I think) one small advantage over phpbb since, as J2EE servlets, all could share the appserver and its session, so that logging in to one is logging in to all.

However, all three used separate user databases and hence separate registration despite being on the same host and being skinned to look mostly like a single application. This resulted in a lot of user confusion, because it was not clear that they needed to register individually on each of the three applications - I received numerous calls from users who didn't understand this.

If you use XWiki's LDAP plugin (and your forum tool has an equivalent option) or a similar mechanism to what we are now doing - which is a servlet filter that digests cookies set by our SSO server and automatically registers users in each one - this will all go much more smoothly, of course.

The filter is simple to deploy but, with very little standardization in J2EE security practices (none that I know of in the user lifecycle space) it can be challenging to adapt to each servlet's specific registration needs. In my case, the generic filter was finished in about three days (counting the time it took to learn the relevant Servlet API parts) but the adapter class that applied it to XWiki took more than two weeks. This was just a little above the average: one JSP developer took my jarfile one morning (while the filter was still in development), informed me of a bug around noon, waited for me to fix the bug and deliver the new jarfile, and had it working with his app before he left for the day; the guy working with Roller took about three weeks (though, to be fair, he was also fighting the vagaries of a new release of Roller with a very different configuration of the ACEGI security package). JForum took about the same as XWiki, mostly because of having to interact with other servlet filters in its deployment.