I never used PHPBB but I use vbulletin because of a few reasons. 1. with vbseo you have the best seo ever (paid program). 2. good community and lots of plugins. 3. Styling never have been so easy. 4. Loads of flexibility and options and most important: It pushes people to post and I still don't know why you just get the urge to post.
I've put up a couple of PHPBB but it always have spammers (robots) entering and posting porn ads. Anyone have experience with vbulletin? Does this happen with vbulletin?
PHPBB 3.0.4 is great. I dare you to install it. I double dare you. Not just because it is free, but because it is 100% XHTML compliant, it runs quickly even on slow servers, and it has many modifications you can install, or that are in development that will be released soon. It is new, shiny, and nice.
In my experience, if you have a big board, the administration control panel is a lot better in vbulletin, more options and more more usability.
If you were using a 2.x.x version of phpBB then that is inevitable unless you are willing to add all the mods. With 3.x.x series, I have found it to be very efficient running and very difficult for bots to infiltrate. Even if you have a problem with bots, there are pages of phpBB modifications available, all of which have easy to follow directions, even if you have not a knowledge of the php scripting language.