Well, the answer depends on you, your skills, and the means and effort you'll be ready to put in the whole site.
It as well depends on if joomla is absolutely required or not. Because you could start from the opposite, and first choose a forum and then a portal to go with it, it all depends.
Anyway, with joomla assumed as mandatory, I'd look at this this way :
If you have few coding skills and if you have not much means for the project, using the "official" board packaged with joomla seems to be the easiest way to go.
The bridge solution is as well good, but you'd be more likely to have to deal with the code, since the probability that a bridge would stop from being maintained is higher than the one where Joomla or a forum script would be abandoned.
On the other hand, if the forum used by default by joomla is a tiny project and / or is barely not used by the community, it could as well be abandoned as a joomla module (in the forum case I don't think this could really occur, but generally speaking, it could).
And in such case, it could end up being a real pain to maintain a deprecated module throughout joomla updates.
I'm mainly telling this to you because this si the main pro for the bridge, because a bridge is allowing you to use to main scripts, thus very likely to be maintained in the future, so this mean that you'd only have to deal with the bridge when one or the other gets updated, and this would really not be comparable to a whole deprecated module update.
The last solution you suggested is not the best one, it's saving a bit work on the bridge, but can only be a solution is you do not use the joomla user base or let guest posts in the forums.
To conclude, since phpBB3 is a great script, and we like it here (no need to tell), it could be worth it if you plan to run a big forum to start getting infos on the eventual joomla phpBB3 bridges, there must be some, and it could be worth it to spend some time on it.
We use such bridges here on phpBB SEO to share sessions across the several application installed, and it has the great advantage to allow several script to be working together while being separate enough for dev, mods and updates.