I've used both, and vBulletin is more powerful for larger sites. It also seems to be liked a lot more by google. Seems the spiders go through vbulletin without much issue.
You'll find very few LARGE forums running on anything other than vBulletin. It's a very stable product, but the renewal price every year gets annoying when you're running a FREE forum with no means of generating revenue through it.
I guess it also just depends on what you want to do on your site as well too. If you just want a plain discussion forum, then just go with PHPbb or a free-bee, but like I said, it depends on what you want to do with your site.