The fundamental principle of submitting test scores, especially to highly selective and top-tier schools, is that they should be good enough to give you an edge, otherwise, you can and should refrain from submitting any. If you have taken both the SAT and ACT, you may choose to submit whichever gets the higher score. But if you have taken both and acheived excellent results, it would be wise to submit both test scores to give admissions officers a good insight into your profile. Great scores on both tests have a positive indicator of your character which could set you apart from the pool of applicants.