Which is better for Data Science, a double major in Math & CS or Physics & CS?
My background is Physics undergrad, Physics grad, now in industry.  Some advice off the top of my head: 


If you can push top 10%, do it (always do your best).  
Don't be surprised when jobs in industry don't care about you ranking/gpa (some do, some don't.). Rank/gpa is  important for grad school however, and grad school is important for careers in data science 
Major in what makes you happy.  
If you stay in physics, learn to code and take some stats.  
Physics will always impress people and there are many machine learning/ data science opportunities in physics.  
Physics will teach you how and where to apply machine learning rather than being given perfect datasets to learn with.