I don't know but I personally like series like, Psycho-pass, steins gate and Guilty crown maybe it's becuse it has deeper stories

I love both but Dark themed anime normally have a better story.

I like both, but my favorite is actually series that feature both themes. I like it when characters go through hardships and life-altering moments that are major elements in the story but also have lots of random fluffy/comedic character interactions that make you want to be part of the group.