I used phpBB 3 a bit a while ago. It has some nice features but (1) SMF is far more established than phpBB 3 will be (i.e. more plugins, themes, etc available -- phpBB 3 is a complete rewrite, after all) and (2) I've used SMF for several years, so I'm quite used to it.
The administration control panel for the version of phpBB 3 that I used was pretty messy, too. SMF has a ton of options and everything is presented in a tight, logical fashion. phpBB 3 offers a lot of options but it seems a lot more spread out to me and less efficient to navigate...me no likey.