does it even matter? i know i would probably get better grades and not have to work as hard this coming senior year if i take band. i can also put down that i was a section leader in it. but i know that ivy league colleges don’t like to see you quit your language if it was offered.
plus does this mean that i will have to take language at Cornell if i get in?
As long as your counselor checks the box that says “most demanding” to describe your course load it does not matter.
In response to your last question, CALS does not have a language requirement. CAS does.
I don’t think it really matters. I was accepted ED to CALS and I did take 5 years of foreign language (Spanish), but the leadership opportunities that you could get in band would also be helpful. I’ve never taken a music class, but have had leadership experience in numerous clubs.
I think both are viewed well; but, if language is considered more challenging at your school, they may prefer that. Provided you are well rounded, either would be fine in my opinion.
My D had both 4 years language and 4 years of band and 4 years of varsity sport and was accepted.
That does not mean I know the answer.
Would it be possible for you to take both? Have you been doing well in your language class so far? What about doing band as an extracurricular? Is there a instrumental music club or something that you could join? That would give you the best of both worlds. 
You application will be viewed as a total package, not just your senior year course selections. If the language will take you to AP, that is an advantage. Can you take both the language and band senior year?