A lot of Afterpay users loved the Afterpay rewards system, which was taken away earlier in 2024 and not replaced. This upset a lot of users.
https://www.reddit.com/r/Afterpay/comments/1fgedmr/so_much_for_those_new_rewards_lol
https://www.reddit.com/r/Afterpay/comments/1arym4e/are_they_coming_up_with_a_new_rewards_system
Sezzle introduced a different rewards/loyalty system which rewards users based on good payment history.
Apart from that key difference, Sezzle also offers credit-building which is so important in the US. Based on these two features, Sezzle seems to be a better offering than Afterpay imo.