Hi guys,
I am currently running a forum with 20,000+ users, and it has been always powered by IPB.
However, I recently started to think about changing to vB, which seems to have a little more functionalities and a bigger modding community.
I haven't ever managed a vB forum and I am calling everyone who has tried both software to explain me his experience and his feedback.
Thank you !!

Both are coming out with major revisions in the coming months so I am very curious which revision will be better. I will watch this thread.

Well, I am and always will be a fan of IPB. :D
But as sammyman said, watch out for the revisions next year, I know IPB are deffinatly bringing out some amazing features :)

I prefer vb because its hook system makes it easy to add my own functions without modifying any code of vb.

I prefer vBulletin and always have done, just a personal choice.
After reading that both are bringing out major revisions, soon, I'll wait and see what happens.