A BBS is eactly that, nothing more, no need to start an argument. Its just that some are more common and have a group of followers who enjoy coding and working with that specific one more.
I prefer phpBB because i have used it more, and am use to the layout and design of the system, folder locations, etc. Its also highly customizable; check out
http://phpbbhacks.com for hacks.
I've used IPB serveral times, hosted at
http://invisionfree.com , and there not my fav. Like with anything, each person has their own preferences to the way stuff is layed out; to me IPB seems unorganized and doesnt have the same "feel" as phpBB.
I've yet to try SMF, so I cant really comment on it, but I have looked at forums runing it, and it looks pretty nice; I have used YaBB in the past however.
Althou I've never admin'ed a VB board, I'm members on a few, and the features they support are very nice. Over-all it seems to be a very nice board; but I'd still prefer phpBB or IPS, since its free.
My .02