A friend of mine had the same dilemma a few weeks ago. Either an Avenged Sevenfold fan-fic or an Original Fiction. Her way of figuring out what she wanted to do, was simply ask herself whether or not she wanted to write a fan-fiction or create something completely on her own.
She decided on Original.
I guess you just have to ask yourself which you'd prefer to write the most. Personally I'd go for Original Fiction.