Any bulletin board system is easy to integrate with time. I personally see no reason to pay for a BB system (vBulletin, IP😎 when there are excellent bulletin board softwares out there already. phpBB3 IMO is not all it could be, but I haven't used it in quite some time. I prefer SMF as it's easier to template (IMO) and I just know that code better. They're working to release SMF 2 (I have the beta) but it should be some time soon they start to release release candidates which should be public.
It all comes down to preference in which code you'd rather work in. Would you want to work in phpBB3's code, or SMF's code? From what I remember, phpBB3 is more object-oriented now and SMF's is still procedural; however, they're making strides to go OO (don't quote me).
Download them, feel them out, then make a decision. Otherwise all you're going to get here is just a bunch of "Oh, I like XX better".
Also, another note on SMF customizations: they include what they call SSI.php* which allows you to log someone in from one page, and have them redirect to another. So it's entirely possible that you could write a script to receive the user after SMF logs them in, and then use their username in the $_SESSION array to log them in again. That is assuming that they're using the same password. Or you can do the reverse, log them in to your site, then us SSI.php to log them in to SMF, then redirect them to whatever page they want to be at (or you want them to be at).
So just download them, feel them out, and also look at what the community has already created. My guess is that someone has already done this for almost every forum, it's just a matter of you finding it 😉
I'm also moving this to Echo Lounge as this isn't really a code issue.
* SSI.php is not just for logging users in and out; rather, it offers a complete "library" to create your own portal to the forum. For an example see: http://www.roundcubeforum.net
