The question is larger than team size
An agency can bring parallel roles and management capacity. A senior independent developer can reduce handoffs and offer direct technical ownership. Neither model is automatically safer; the right choice depends on scope clarity, number of disciplines, urgency, governance and the buyer's ability to make product decisions.
When direct senior ownership helps
A focused system, backend, automation or recovery project often benefits from speaking directly with the person reading the code and making the change. This model works well when the work can be sliced into reviewable increments and specialist needs such as branding or security testing are identified explicitly.
When an agency is the better fit
Large concurrent streams, intensive discovery, brand production, round-the-clock support or formal account management may require a structured multi-role team. Ask whether the named specialists are actually allocated and how knowledge survives team turnover.
Questions that reveal delivery quality
Request examples of acceptance criteria, risk reporting, test evidence and handoff. Clarify repository ownership, access, communication cadence and what happens when an assumption fails. Choose the model whose operating behavior you can verify, not the logo or hourly rate alone.