The verdict
For a small business that wants to run follow-up and payments in one place, I'd pick Keap. It is the simpler starting point and does not depend on another platform.
I'd pick Pardot when the company already pays for Salesforce, wants marketing and sales on one set of records and has enough lead volume to justify a dedicated tool.
Pardot is sold on annual contracts, priced per organisation, and the entry edition costs a lot more than tools small teams usually start with. Keap has one plan priced by contact count and an early termination fee on annual contracts. Neither is the easy option for a team that wants to start cheaply. A CRM with marketing built in, such as HubSpot, is a lighter alternative.