The verdict
For a small B2B or service team, I'd pick Keap. It holds the deals, the bookings and the payments, and Omnisend has no sales pipeline to build on.
If you run an online store, I'd pick Omnisend. The flows start from shopper behaviour, the migration service moves your lists and templates from another platform, and the free plan lowers the cost of trying it.
Check how each price grows. Keap has a single plan priced by contact count, with an early termination fee on annual contracts. Omnisend plans also scale with contacts, and SMS is billed per message at rates that change by country. Estimate your list at today's size and at double.