If you have to install a forum in your site. Will you use wordpress plugin or forum script such as phpbb?
Scripts are better because they comes with a lot of options which a community needs.
Use script even if its free. Still it will work way better than any plugin.
Scripts are better that's totally true. If you don't have much money to buy a paid one then go for a free one such phpbb or mybb.
Why do you wanna use plugin in the first place. They don't work well most at the time when the forum starts growing.