SAT subject tests or AP exams?

I am currently doing my A-levels in England and I wish to apply to universities in the USA. I have taken actions to taking the SAT but I am thinking whether to take the SAT subject test or AP exams as an extent. I heard that the SAT subject test is more high school based while the AP exams are more college-based, so does it look more impressive if I have the AP certificate?
And most importantly, if I take the AP exams and I get a good grade like a 5, is it that I don't have to take the corresponding SAT subject tests when applying to top American universities like MIT and Harvard?