Boxing is more about violence, whereas karate-especially in the style of Shotokan is well known to be associated for Self Defence.
I am currently a first dan black belt in karate, and I can say that I feel confident when I spa (fight) with another person in the training room, and again I can say that I feel safe going out anywhere because I feel that what I have learnt over the years in karate (by basic techniques and fighting skills) has taught me to be safe.
I thin karate is better self defense because you learn how to use a lot more moves than boxing. Boxing it is just punching, but karate you can kick, head lock, punch, jump kick, and also learn the mental aspect of self defense. Go with karate.