SAT II Bio or Physics?

I’m taking AP Bio this year, and I think that if I take the subject test this May (just prior to the AP Exam) I’ll have a good chance of getting an 800. However, I want to go into mechanical engineering in college. If I’m applying to MIT, for example, should I take the subject test that is more relevant (physics), and get a lower score since I’m taking AP Physics my senior year, or should I take the subject test that I’ll do the best on (biology).

I don’t know what advice others will offer, but I want to go into aerospae engineering. However, I’m only in AP Physics 1, which doesn’t cover enough material for the subject test. I’m also in AP Chem, so I’m just gonna take the Math Level II and Chem subject tests.