I do maths as well as music at A level. In my opinion, you should take music if you:
-want to go into depth on music in different time periods; western classical, jazz, 20th century music, rock
-have interest in the history of music, e.g. how jazz music, symphonies are developed
-are willing to learn about the background information of composers from different time periods
-want more opportunities to compose and improve your composition skills (if you found GCSE composition easy, that's because there were basically no rules on what you can and can't do in your composition)
-want to improve your aural and dictation skills
Basically, don't choose music for A level purely because you found it easy and enjoyed it at GCSE. I got an A* at GCSE but revising for A level music is A LOT of effort because I have no interest in the things we're learning.