In general analysis over the years without data to back it up.
For big name private research schools, the average MCAT and GPA for the MD/PhD program is not that much higher than that of the medical school.
For less highly ranked private schools, the average MCAT and GPA for the MD/PhD program can be much higher, assuming full-funding. It depends on a few factors I'll list below.
For state schools with fully-funded MD/PhD programs, the average GPA and MCAT can be much higher.
Though when the MD/PhD program is not fully funded, it can be as competitive as the medical school.
Factors influencing competitiveness:
1) Money. No funding = no greater competition.
2) US News Ranking. Given that they're based on MCAT scores, GPA, and "prestige" this is then a self-fulfilling prophecy/feedback loop. But, it is what it is.
3) Location. In broad terms 20somethings tend to prefer large, coastal cities. Midwestern schools are less competitive in general.