Hello there,

Hope you are doing well. See, at the end of the day which subject you take as an optional won't matter . In order to appear for both JEE and NEET and do well in both ,you need to be proficient at both the subjects . So, you'll have to study your optional subject with the same regard as that of a main subject. You'll have to study both the subjects well . So, it's totally your choice . Eventhough many students are there who get good percentiles at JEE Mains level by doing Excellent in Chemistry and Physics but doing Maths is also going to help you understand Physics . So,it's totally your choice that which subject you take as an optional. If you are more inclined towards medical ,NEET is your exam and you can take Mathematics as an optional and if you want to pursue engineering after 12th ,JEE is your exam and you need to be proficient at Mathematics and in that case you can take Biology as an optional.

Hope my answer helped.