As a longtime swimmer, I can definitely say that breaststroke is generally slower than freestyle. However, it may be easier to do for a longer period of time if swimming isn't your strong point, and the breath control is easier if that's a problem for you. Like a couple of previous posters, I'd be concerned about kicking somebody in the head if you're doing breaststroke in a tri--it's not fun to be on the receiving end of that (I've experienced it in crowded swim team practices once or twice). I'd say the best thing would be to practice your swimming so you get better at freestyle. Otherwise, maybe try to position yourself at the edge of the group during the start to make it less likely that you'll kick somebody.