The verdict
For a small B2B or service team, I'd pick ActiveCampaign. Lead scoring, task creation and a CRM in the same account are what such a team needs, and Omnisend is not built around leads.
If you run an online store, I'd pick Omnisend. The free plan lets you send real campaigns before you commit, free migration moves your flows and templates, and the abandoned cart and welcome flows start working quickly.
A store with a long B2B sales cycle on the side is the awkward case. Run the store messaging in Omnisend and your sales follow-up in a CRM-led tool. Check how both prices grow with your contact count before you move a large list.