It's a tough question. We moved away from phpbb due to spam
issues a couple of years ago, and when invisionboard went pay-for we switched to moodle for our work.
The most important thing when considering the switch is to try not to see the moodle forums as a single entity. They are, by their nature, part of a greater whole, and a simple comparison of the features will, i think unfairly, prejudice you against the moodle forums. The tracking, for instance, can be done in various ways, not immediately apparent by looking at the forums in isolation.
The best thing to do if you'd like a precise comparison is post the things that you would like to be able to do and maybe we can explain how that would be accomplished with moodle. Some of the functionality might be better accomplished in the assignment module or somewhere else...
As for replacing the moodle forums with phpbb... it may be technically possible, and i leave room to be corrected by those here more knowledgeable... but i wouldn't consider it as an option.