Whichever one you can write more interestingly about. EC essays’ effectiveness is not predicated on the activity itself but rather on what the applicant can say about it, what comes through in the essay–it’s you applying, after all, and they’re reading the essay to learn something about you. They already know about pianos and research–they want to know about you. @JoeSmith15
Write about whichever means the most to you. How you write the essay, how the essay tells your story etc. is way more important that what you write about. Your publications/awards etc. will be on your application so you can also feel free to write about an entirely different topic…