Sequel...or Prequel?

I'm having some big problems. Because I like going ahead and having things in advance, I am currently writing my second novel. The only problem is...is that I don't know whether I should write the sequel...or the prequel. Prequel's are sort of like side stories from a different character's point of view. Sequel's are the leading up book to the author's first debut.

But for me, I'm not sure at all. After I finished my book I planned that I would write the prequel, but now that I have some good ...