I used both cms vbulletin and xenforo. According to me, vbulletin is easy and more plugins than xen.
what about you ? which cms will you choose?
The first thing to do would be to run the numbers to see what the pricing for each one would be for you:
Also once you’ve looked up the initial price and any repeat license fees for either, you need to justify to yourself whether you will get enough traffic and if the forum will get big enough (number of members, number of threads and number of posts) to justify the cost of either of them.
Would an open source one suit your needs better?
What else does it have to work with (eg for example will you be setting up the forum so that when you log-in to the forum, you get logged into other sections of your site at the same time)?