I'd say post-bacc, because those grades will count in your undergrad GPA and help boost that up, unless you're getting a masters from like Midwestern or something, which are known to be feeders into dental schools. I like post-bacc though, and am doing an informal one myself, where I'm just taking additional "recommended" courses through a nearby university. So I won't be getting a certificate or anything, but will get grades from a top-notch school on my transcript.
Good luck, and let me know if I can answer any questions!