I prefer vBulletin. It's easier to find themes and other things, also it's easier to setup. Both are very good in my opinion, which makes it hard to choose, but vBulletin looks cool. My choice is vBulletin most of the time
I participate in the staff of a site with vbulletin, the integration is very complex from one version to another, the portability of modules is almost impossible, I do not know if it is a good idea for those who want to create pluugins and modules for vb and then update.