Having chosen Btec applied science over biology, I do not recommend taking the Btec over an alevel. Btec was previously regarded as the easy option however due to the new changes it has been made equivalent of alevel. So if you were going to learn all the Btec content why not just pick the alevel. Also I assume you will be taking this course starting in september and Btec qualifications will have an exam on a unit, which will be essentially the same content as the alevel. However it depends on what you are comfortable in, check university requirements and what they prefer ring up or email. Its worth speaking to a teacher/careers advisor.