I don't know if there are any stats on whether a post-bacc program, PhD or work experience would make a candidate more attractive -- I imagine, though, that it really depends on the individual situation. For example, a post-bacc run with a 4.0 gpa would make someone more attractive than a 2-year stint as an anonymous lab-tech, but a job in a lab with a well-known researcher who writes you a stellar recommendation would blow the pants off a post-bacc with a 2.0 average... See what I mean?
Like was said before, it depends on your situation. If your grades need help a bit, the post-bac is a good idea. If you have good grades and you would like some great experience, then a lab job might be fun for a short while. If your MCATs stink then you need to study more for the MCAT. Personally I did the lab thing and am now in a post-bac program. I got a great letter of recc, but no one letter is going to get you into medical school.
