DT would be preferred by admissions, I'd guess. Music is good though and if you enjoy it, do it.
Try and do an AS in Further Maths, it will help your application greatly. Good luck =]
A bit of background -
I'm applying for Cambridge. Did Maths, FurtherM, Physics and Chemistry for AS, dropped Chemistry for A2 this year.
I'm hoping they'll still like me. They were all 96%+.
Fingers crossed. =]