No doubt klaviyo is the best tool for email automation. MailChimp is for email delivery. Klaviyo is making money for you. Klaviyo is designed for e-commerce stores from the ground up. Integrations of Klaviyo get in more data. Automations for Klaviyo email are easily customisable.
I tried a number of services and found out Klaviyo is one of the best email marketing services available.
Mailchimp has been a long established brand in the email marketing field. A fantastic tool that is easy to use for those just getting started with email marketing, if you’re serious about email marketing Mailchimp is probably already on your radar.
MailChimp API is very easy to navigate, and you might have an app catch an item in an order, then dynamically create a merge tag per customer.
I migrated to Mailchimp from Klaviyo but I am considering moving back.