Reading your question , you have a lot of chances to get a seat in Mathematics/CS. As you didn't mentioned the specifics about the colleges between which you are confusing , I have no way to give you info regarding that.

Now the 2nd confusion is regarding the selection of the subject. So the selection of the subject is entirely dependent on the interest of yours. Now as you told that you are good in both Mathematics and CS so you will have to choose between these two. Now I think you are probably thinking that both the subjects you are interested in but most often what we think aren't absolute true. So you have to prioritize between these two subjects which can only be done by you with patience and self questioning. In this process no one can help because Its a very personal thought process which you can only be sure about.

So don't waste more time just think what you like to do. And just pursue that.

I hope my answer helps you. All the very best for your future endeavors!