The sequel. I bought it last night at BAM after I finished Life As We Knew It and read it today. It's not nearly as good as the first one. I paid $17 for it and I'm returning it tomorrow. I almost feel bad about buying a book, reading it, and then returning it, but if it had been any good, I'd keep it.
My recommendation: Definitely read the first, and don't waste your time or money on the second.