everyone has their own aptitude for different subjects. i took both chemistry and ap calc ab. for me, chemistry was easier because it required simple math. i hated calculus because i didn't understand it and in my opinion, my teacher sucked at teaching.
give calc a try.if you plan to go to college, you'll have to take it anyway. although some may disagree with me, a lot depend on the teacher. it's difficulty also depends on what you're good at. science or math? to me, i hated both.
chemistry is a normal hs science and ap calc is a college level course. unless you got straight As for all previous math courses, i would say ap calc is harder. not by much though. as long as you have good previous knowledge of the material (functions, factoring, inverses, trig...)