I started my forum with phpbb. After migrating to vB after two years, search engine traffic doubled in a few months. This alone is a deciding advantage of vB.
In case any dev reads this: make vB even more SE friendly. Dump some other features if you have to.
That was written back in 2005. Do you think it still applies? If we switch to vbulletin here, would search engine traffic double?
vbulletin forums do seem to show up higher in search engine hits than phpBB forums. Why is that?