PHPbb or vBulletin

I am planning a forum. These are my primary prospects phpbb and vbulletin. phpbb seems to be free and has cool features and vbulleting you have to pay for but seems to have cool features also. My question: Which one is better and are there any pros and cons that anyone has experienced to using one over the other.

Also I will most definately be skinning it. Is one easier to skin over the other? Any advice and user experience greatly appreciated.