I would say a Master’s in CS (I’m a CS major). This is just coming from a data science perspective (just one of many concentrations in CS). It really all depends on what you want to go into in the CS world. In data science, most data scientists need some form of graduate degree. Even if you double majored and has a lot of math skills that is needed for data science, employers just like that extra certificate.
Secondly, CS already has a lot of math required. Double majoring will make employers think you know a lot of math, but having a CS degree alone also has the same effect, they will still think you know a lot of math (because CS curriculums need lots of it). So double majoring can be a great way to push yourself intellectually, but you should look at your special interests to see what kind of job you’re into and look to see what employers would prefer for those positions. Some positions, a regular CS degree is sufficient. Some would want a little something extra. Do ones that fit your interests.