I am a Xenforo user and while what sebastian says is true, it is something to thread with care. $140 is the cost for the license and one year of updates and customer service, after that every year is paying $50 dollars if you still want to get updates.
Other thing I found the hard way with Xenforo is the add-ons, in MyBB many you find them free, in Xenforo a lot of those add-ons might actually cost you and some might mean a yearly license fee. So if you go to Xenforo and want to pack things you need to be ready to shred more than $140. That said the software doesn't really 'need' any add-ons and those are just to increase functionality. Also there are some free add-ons around, so it is not like they are all paid ones.
another thing to consider is that to convert the database you might have to pay or find someone willing to do it. Also as a small possibility it could mean starting over if you can't convert for any reason.
Lastly, by changing software it means all the staff would have to get used to the new interface and the way Xenforo does things. You might miss things you take for granted here.
The way I see it though is that as it is you're already struggling to get by with the community, I don't think you should get another debt on top without thinking in through. However if you guys ever go through this, I am available if you need any kind of assistance to handle the admin interface after the transfer. I moved from MyBB to Xenforo and besides what I mentioned for the software itself I have no complains.
