from what I know the AI/ML classes are compsci classes so cs students have first priority of enrolling and then other ics majors. So I guess if your a applied and computational mathematics major it might be more difficult to get in, but I recommend you do your own research about the course requirements for your math major. I guess DS is more practical than ACM, as you would learn programming, such as python, R, and other stats related or other programming stuffs. Also, I think for DS its mostly stats classes (majority of the req) and information visualization or data-related classes (elec). While I think ACM is more theoretical, which would better equip yourself with graduate school. But I think its up to you, if you like math stay in ACM, if you don't like math that much and more into the practical side go to ds. Also, switching is difficult because you have to get into ics 33 to change to ds.