Splitit has been consistantly improving the merchant base better than AfterPay as per the merchant plugin installation statistics (Active Installations Rate and Downloads/day stats) for Woocommerce (both provides numerous ways to integrate APT and SPT in the merchant sites, WooCommerce is the Wordpress based ecommerce platform).
This is not a very accurate indication of the expansion as there are 4-5 ways to get the implementation done, such as Magento, Shopify, direct SDKs etc. But this indicates the statistics just for the merchants who are using WooCommerce.
APT
https://wordpress.org/plugins/afterpay-gateway-for-woocommerce/advanced/
SPT
https://wordpress.org/plugins/splitit-installment-payments-enabler/
Stats for SPT also indicate DOWNLOADS PER DAY graph showing a consistent rate, end of 2018 and early 2019 stats show a good number of plugin downloads which will be indicated in the coming reports (merchant growth).