We've had discussions about this in long and broad, but I would definitely recommend IPB over vBulletin. I've been using it for a long time without problems. In comparison to vB; It costs less, supplies you with more features and flexibility, and it doesn't force you to renew your package (i.e. if your license expires you can still use the software, but without upgrades or support). Renewal is $25 every 6 months.

The admin CP is also very user friendly. For example if you want to re-arrange forums you can do so by simply clicking and dragging them in the order you want. Changes reflect immediately without the need to click additional buttons. You can also restrict ACP access for some other user groups. Useful if you have moderators that need access to the forum logs or member's accounts, but not the general settings. Also supports live search in the ACP, so you don't have to look high and low for a simple setting.

You can also download some free apps, skins and more at their online marketplace. You can for example install IP.SEO, a feature you have to pay for at vB.