I plan to major in physics, and to apply for competitive schools. I will be taking Calc BC senior year (I’m a junior now), and I would like to know whether I should self-study for Ap Statistics. Will it be useful?
AP Stats is considered one of the easier APs so it shouldn’t be that bad to self study. Self studying doesn’t really mean much to adcomms, as I hear. The real question would be then, will the school’s you are intrested in applying to accept it for credit/placement?
AP Stats is very easy! I’m taking it now, and I wish I took it by myself. It’s a very helpful class because you can actually apply it to real life. If you’re worried about how it will be hard taking both at the same time, don’t be. It is a very easy going class with just some reading. The hard part later on is knowing what kind of test to use (z-test, etc), but it shouldn’t be too hard to self study 
But, you should see if you would get credit for it.