I would go with vBulletin 4 if it were me.
Once they invest and redo the plugin system to where its back to its former glory days we saw in vB3 and vB4 then you'll see more modifications and plugins come out but imho they're moving in that direction *cough *cough vB Cloud but who knows when we'll see that and until then you'll be directed here for outside the box vBulletin questions and sadly we can't offer much support for vB5 when the plugin(what some call hooks) system is not up to par or well sorta non-existent in a sense... basically some simple mods and changes using plugins are not possible in vB5 when they were in the past in previous versions, lack of a decent plugin system is the cause of this.
Follow the link in Ozzy's signature to vote for the plugin system to be completed and re-included into vB5 -
http://tracker.vbulletin.com/browse/VBV-10333