I know not everyone is in the position to spend close to $200 on a forum software, but if a forum is going to be a part of your site for the long haul, I think it's important to think about which forum software has the features, upgrade frequency and support to keep you a happy customer for many years. My last experience with PhpBB was with the 2.x series (not the new 3.x), but just between that and vBulletin, there's no contest which one is better- vBulletin. Phpbb's control panel is severely lacking in features, a lot of them crucial ones. Customizing Phpbb's templates require manually editing text files. And the final straw is how much Phpbb forums get spammed and the lack of spam fighting tools. It got so bad I basically threw my hands up in the air, dropped the entire database, and installed Simple Machines
instead (this is for another site).
Again, I'm not sure how much Phpbb has changed with their 3.x series. Might be a completely different beast than its predecessor. But I can vouch for vBulletin.