I have an MPH (George Washington Univ 2001).
I wouldn't recommend getting an MPH solely for the purposes of getting into dental school. I don't beleive it may be the best way for you to "bolster" your record for dental school admissions purposes:
1) MPH curricula gernerally don't have many science courses, therefore not many chances to increase your science GPA.
2) Unless you plan on going into dental public health, the training you'll receive in MPH won't be very useful assuming you get into dental school.
3) Assuming you don't get into dental school, you will be left with a degree in a social research field. If you don't plan on going into research as a back-up plan, then the degree is of no use to you.
Having said this, getting ANY graduate degree is better than nothing and getting an MPH will look better than applying again with just your bachelor's. If you DO plan on going, I would recommend going to Emory (of the 2 you mentioned):
1) Emory is situated near the CDC. Great for research/job opportunity.
2) The Rollins School of Public Health has the most resources for students of all the MPH programs I've seen