Hello,
Personally I think you should take Music. I don't take either subject, but I do know a lot about them both.
In Art you will have to do a lot of course work, which is very time consuming, especially when you need to be revising for mocks and the real exams. I loved art up till year 9, but I knew that it would take too much time, and in the end stress me out. If you are a fairly quick 'artist' (unlike myself) then it probably won't be too bad. However, a lot of my friends that did take it feel as though it takes a lot of time out of their day, considering they need to finish 3 A3 books in just 2 years.
I think music would be the best option, firstly I would make sure that the teacher is good beforehand, because my friends that took it are now going over everything in a few months, so they are not going over things in much detail, because they had a bad teacher in the first year. Whereas with art, it is a lot more independent work. To take music I would definitely say you need to be a grade 5, and preferably with a teacher outside of school. A lot of the current music course is composition, and 1 test at the end of the year, which sounds fairly good for a GCSE test.
Sadly the course is changing, Im not sure how much it will effect music or art, but the only difference would be is that there are 1 or 2 more written tests at the end of the course.
Definitely ask your teachers about what they think, normally schools have a parents evening before the options are final. Its a good opportunity to ask then. Else just ask during a lunch or break time.