There's some sort of 'Urban defence' class going on near where I go to boxing some days, it seems pretty practical... but they never actually get to practice at full speed or fully complete their movements (because it's designed to be used quickly and to totally disable an attacker I guess) - which makes me think it's totally pointless - if you can't practice at full speed or full force you will never be able to produce it at crunch time.
Boxing is much better, a decent gym will have you sparring very soon after you join. When you've felt the pressure of the ring a few times, you quickly learn to deal with it, and I think that clear headed-ness and experience of danger/pressure would work much better if you got attacked for real.
