I honestly think they are both very comparable and you will not find many major differences. I think the only real major difference between the two is phpBB has a much larger following, which could ease your pain some on getting a forum going. Cause it is a TON of work to get one flowing and working properly with enough traffic per day to not loose peoples interest.
Why not try this simple test. Do an A/B variable test, setup your forum on both use it some and see what you like and do not like about each one. Or pick one launch that version then on a subdomain launch the other one and offer users the ability to go back and forth between the versions. and make them vote on the main site which is the better one.
Either one you will have to learn the software some at the very least. These forums run on myBB and you seem to use them a bunch so I would think you might be happier with myBB at least in the beginning for the familiarity.