Since when does phpBB have more exploits...? There are no currently known exploits, and even if there were, they would be privately owned exploits, and in which case, your site would hardly ever be the target of such an attack.
I say go with phpBB3. It's (more) widely used, many free skins are available, many modifications, and it is easy to get a hang of. If you know PHP, you can also easily develop for it yourself. (You can do it with SMF, just easier with phpbb)