I've tried both,

chose for phpBB in the end, because:

1) phpBB is GPL, you're allowed to modify whatever you like, never have to worry weather the code changes you make are legal
2) better support (personal experience only)
3) it's free :)

It seems that IPB has some better moderator features for large boards, but I must say I didn't notice myself (probably because I don't run large boards ;) ).