I guess, as previous people have said, it all depends on what you need out of a forum.
I babysit a VBulletin forum and I have to say, for what we need it for, we could just as well do with SMF. However, someone else pays for the license and everyone's used to it so that's all well and good
Another point to consider for some would be that many popular Forums are backed by VBulletin so perhaps for the sake of usability, you might want to "go with the flow" and make life easier for visitors that might be members of other popular forums. Just a thought.
Androo