A lot of people are choosing to go with XenForo because of several reasons. Pricing is one of them as well as features, customer support and several other reasons.
XenForo was actually created by a couple of developers that were part of vBulletin. vBulletin is losing some of it's market share to XenForo over the last couple of years.
I have used both of these softwares from different roles and I have to say that they are both good and both are good at some things that the other isn't. It would really come down to preference, pricing and support needs for the user of the software to decide which they like and which would fit their needs best.
I personally prefer XenForo, but I would be happy having a forum built with either.