My advice would be look at the various forums and decide what you want/need.
What does phpbb3 give you that Kunena doesn't?
How many visitors are you expecting to get on your forum?
Do you want a forum that integrates perfectly into Joomla?
Do you want to spend any more money for phpbb3 templates?
Would you prefer the installation to be straightforward and easy to understand?
These questions will determine whether you opt for phpbb3 or Kunena. I was a member of the PHP BB three club a while back and created a website using matching templates. My honest opinion on what is better, PHPBB3 or Kunena?, Kunena wins hands-down in my opinion. Don't get me wrong, there is nothing wrong with phpbb, however the installation is nowhere near as easy or straightforward as Kunena. I also had great problems in getting the matching RT templates to work properly. Once everything worked properly, everything looked great, although you still didn't have any Joomla navigation on the actual forum.
Whereas my website gets quite a few hundred hits a day, not everybody is on the forum at the same time so it seems to be able to handle this okay, I'm not so sure how well it would do if you had 500 people on that same time. This might be where phpbb3 comes into its own , although the Kunena forum themselves don't have the problems, and they often have quite a lot of people on at the same time
I've just installed this on a test website. I hate to say this but it seems like it's easier to setup than Kunena, and Kunena is very easy to setup. It also gives you some functions that you will find on phpBB such as a poll which is something that I really like. It also seems that you can create custom meta heading and description for each forum, that is something phpBB doesn't have. In fact, I think the phpBB forum and thread headings absolutely suck Kunena is 10 times better, it seems that that Agora have gone one step further.
I haven't delved too much into it but I think I'm going to install it on my personal website which hasn't got a forum at the moment. Quite excited, this looks extremely promising. Kunena better get their act together because this looks really good
Agora is a piece of cake to configure, It took me about half an hour to create the categories & forums, create an API for the Google map, set up my profile etc. It picked up all the users in your Joomla database, or if you had any other forum installed previously . Anyway, check it out, join up and test, I can always delete your account when you are finished.
I prefer vBulletin for all my forums, and MyBB if you're looking for a no cost alternative. The main problem with scripts like phpBB and SMF would be the lack of functionality compared with vBulletin. You may of course use mods/hacks to cover some of the functionality you find in vBulletin, but then you got the potential security problems that would rise with the hacks.