WordPress actually has a forum plugin - it's
nothing fancy but it's there. It handles user management et al and
has a big community for plugins and themes. I think it is probably
the easiest CMS to install & run (I've done some legwork here).
There are plugins that update the core & plugins automatically
(take that Drupal). I've tested these and they seem pretty solid.
As usual - backup beforehand.
Drupal is a language unto its own - I wouldn't
tackle it unless you're going to do so with some regularity,
otherwise it's just another different framework to learn. The
uplink into my brain is at capacity already so I gently pushed it
aside. The themes tend to look the same too.