I assume you're talking about in an MMA fight. It really depends on how you feel about your skills. If your hands need work train boxing, if your kicks need work train kickboxing.....The better trained fighter would win in a fight.
In my opinion; training kickboxing would be better for MMA because you have 2 more limbs your opponent has to consider; and it allows you to mix your striking up and throw fight ending head kicks
but; do what you want and want what you do. Both styles are great.