The verdict
For a small team starting out with email and simple needs, I'd pick Mailchimp. Nobody needs training, the free plan keeps the cost down at first, and it connects to most small stacks.
For an agency sending for several clients, or a team that wants on-brand emails and clearer reports, I'd pick Campaign Monitor.
From setting Mailchimp up at several B2B companies, I know where it ends: automation feels rigid for branching paths, reporting stops at opens and clicks, and cost climbs with a large list. Test the same scenario in each trial before you move a big list.