A-Level: Music vs Music Technology Watch
Although I don't know much about music technology by comparison, at my college it was taken as a BTEC and I think it was all coursework - you are graded on modules taken throughout the year with either 'pass', 'merit' or 'distinction' and this constitutes whether your overall grade is one of those 3. I would guess music A-level is a little more taxing, because you are tested on various areas, but it really depends on what you want to do. While I enjoyed music A-level, I wish I could've taken music tech now because I want to produce my own music and this would have helped massively.
While there is some recording involved in music tech, which involves performing, you will do more performance for music A-level since you are tested on it. As for your skills on cubase, this would help but they don't assume you to have any skills and you may be using other programs too, like Logic.
tl;dr It depends on what you want to do - Music A-level is probably more academic and you could take it on to Uni while music tech would help with recording and producing music.