Once you get closer to average ranges for actual accepted applicants, I would say MCAT matters more because I think in general there are more unsuccessful applicants that have high GPA/low MCAT than have low GPA/high MCAT... few people can make an amazing score on the MCAT without a lot of studying, and if they were willing to study that much for the MCAT, their GPA is probably pretty good too. On the flip side, plenty of people with near perfect GPA's just struggle with the pressure and format of the MCAT.
As an example.. the national average GPA for applicants is like a 3.5, if you are an applicant with a 4.0, schools will say 'wow, that's good!'.. but GPA's are not standardized and majors differ etc.. The national average MCAT for applicants is like a 29, if you have a 38 MCAT most schools nationwide will keep you on their desk for a long time (It's standarized.. and they see your test score and think those will carry over to boards, etc. and this probably holds more weight than a 90th percentile undergrad GPA) . I realize that this is totally theoretical as EC's, experiences, and the combination of the two scores is really what matters.
These two cases are not realistic because a 27 MCAT in sam and a 3.0 GPA in Teddy will pretty much get them filtered out from the beginning.