Freelancer: senior attention at $50–150/hr, no account-manager markup, one channel deep. But bus-factor of one (vacations, capacity, disappearing acts). Agency: continuity, multi-channel coordination, process. With your work often executed by the junior staff the pitch never mentioned. Rule of thumb: one channel, hire the craftsperson; orchestration across channels, consider the agency.
Technical requirement to manage either: nearly none, IF you hold the right five numbers (leads, CPL, close rate, response speed, reviews) and insist reports speak that language. You're not reviewing their bid strategies. You're comparing money out to customers in.
The non-negotiable literacy is ownership: knowing which accounts exist and that they're yours. Our audit tool covers that in two minutes, no jargon required.