Math vs Math Education
As far as I know, university mathematics departments with any kind of significant research focus do not hire people who have a math ed. degree.  You need a math degree if that's where you want to work.   Even the education option will only hurt your chances of getting a tenure track job in a research-oriented math department. 

If you do some searching, you'll find that people with math ed phds are generally employed in education departments rather than math departments.