It does depend on many different things like age group of your audience, features that are required or like to have, your ability to create the extras that you want etc etc etc.
Many of the free software versions become popular simply because they are free and therefore become the focus of hacking attacks...paid for software less often.
Forum software is a business and the internet is littered with failed sites, empty forums or forums full of spam and crap. To do it properly you need to be aware of the forum software industry, know what is getting developed, what is up and coming and why, trends in technology, hosting administration, html/php coding conventions etc OR you can install a forum software app and hope for the best. For example many many many of these forums that are installed by non-IT people are continually hit with spam driven by the X Rumer forum attack software, software that even beats the Google recaptcha simply because they install a forum software app without knowing that running a forum is extremely hard work in ensuring that it runs properly. Note, I developed our own anti spam system here on Recreational Flying which is why we have never been attacked by X Rumer however many other sites are attacked on a daily basis.
The point I am making is the first part of installing a forum software app is making sure you select the right one and 99.9% of the time the right one WILL cost you money, and secondly, once you have installed the forum software app the work you need to do is only just beginning.
(don't forget the legal aspects of having a user integrated site...I spent a lot of money getting all this right not only in site rules but also the makeup of the behind the scenes ownership etc)
Ring me on 03 9444-8025 if I can help