?
For what purpose?
You indicated in a previous thread that you’re a freshman in high school, so you have a lot of time to decide. In your first year or two of college, you can take classes in both math and computer science. Once you’re an upperclassmen, you’ll have more of an idea of your career goals and, if you want, you can narrow your focus to either math or computer science. (Of course, by the time you get to college you might be interested in other graduate programs or another subject entirely. I’m a math major now, but that wasn’t on my radar at all when I first started high school.)
With the computer science degrees, it would be more straightforward to get a job. The math Ph.D. might be a good idea if you’re really interested in math research and understand that the academic job market is terrible.