You want to know which is in more demand? I guess I am not qualified to tell you. However, part of your preparation is supposed to involved reading pertinent research studies, so you probably will be able to find out.
Your topics are very broad, and your question about which is in more demand is very broad. I think you should read some articles about each, and just really enjoy them. Take your time, go slowly, and wait for your research idea to take form.