When you don't care something use 我不在意。That means nothing is important to you ,you are not interested in it! But when you say 我不介意。maybe you care.but it's ok for you. For example, If in a relationship, my bf may say "我不介意",but he still love me and care about me. But if he say "我不在意“,he doesn't care and he doesn't love me any more. That's the difference,hope it helps.