I've tried them both, and I'll never run another phpBB system again.
For one thing, phpBB has tons of hackable holes. Tons and tons. I've seen way too many phpBB systems hacked and flattened to ever trust the software. SMF has relatively few. If you veiw the security advisories for both boards you'll see that phpBB has many more vulverabilities, some of them months old.
SMF has a lot of features, at least as many as phpBB if not more. A lot of the features are turned off here, but you can go to the SMF site and see a full list.
SMF is also written very well in my opinion. It has a very good administrative back end
and a lot of 'anti-trolling' controls built in.
Just my 2¢.