The verdict
If you have no email tool yet and you need a newsletter or a simple drip sequence, pick Mailchimp. It is familiar, quick to learn and has a free plan to start on.
If you already send email and the problem is thin leads from a busy website, pick Outgrow. An ROI calculator or a readiness assessment gives the visitor something useful and gives sales answers they can act on.
If you can only afford one tool, I would start with the one that fixes your actual bottleneck. No contacts to email means Outgrow. Contacts and no way to reach them means Mailchimp. Neither wins for every small B2B team, which is why I leave the winner open.