Before matriculation is best. 😉 But in all seriousness, its best to decide sooner rather than later so that a student on the waitlist can have your spot.
If you’re having a hard time deciding, write out why you want to go to each school. Ask others for advice—friends, family, or a neutral third party. If you’re still in school, an academic or faculty advisor is good for this. Give yourself a deadline—say, two weeks, and if you don’t have a decision by then you’ll flip a coin.