This is like comparing apples to oranges. Imported fire ants and killer bees are both very aggressive social insect species that attack en masse when they perceive a threat. Rarely, each can cause fatal allergic reactions. Africanized bees, which are a variety of honey bee, can overwhelm and kill even healthy, nonallergic people, but such encounters are rare. Fire ants can't overwhelm a healthy, mobile person, and even hundreds of stings are rarely fatal. Africanized bees are probably more dangerous when encountered, but you are much less likely to come into contact with them than with fire ants. The bottom line is that your chances of being killed or seriously injured by either Africanized bees or fire ants are very low.