In my honest (and personal mind you) opinion...
vB4 > Xenforo
vB5 < Xenforo
vB3 < OR > OR = vB4 (it's a Tomatoe, TomatO, Potatoe, PotatO type of deal)
vB4 > vB5
That is based on the current versions of each, the most recent. I do expect to see vB5 improve vastly over time, the only thing it can do is improve unless they spend more time and resources into other ventures such as the cloud. I do not like the idea of the cloud for experienced forum owners but for those new to vBulletin it should make things super easy for them

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