I've heard the Qvault's Bio is pretty helpful, but I didn't end up getting it. Rather, I bought OC because that's what I needed the most practice with. I think it really depends on what your weaknesses are. I found Kaplan's online material to be more than enough when it comes to review/getting down the basics. But if you want different material just to cover your all your bases, as well as UI that's more similar to the actual DAT, then go for Qvault.