Short answer
Use a consultant when you need judgement, prioritisation and governance. Use an agency when you already have clear requirements and need delivery capacity. Many firms need both at different stages.
When a consultant is the better fit
A consultant is useful when the problem is not yet clear. They help leaders decide what to automate, how to manage risk, what success should look like, and whether the firm is ready to build. This matters when partners disagree, teams are already using AI informally, or the firm handles sensitive client information.
When an agency is the better fit
An agency can be useful when the scope is defined and delivery capacity is the main constraint. If you already know the workflow, systems, budget, acceptance criteria and risk controls, an agency may help build faster than an internal team.
The mistake to avoid
The mistake is hiring delivery capacity before the firm has made decisions about governance, data boundaries and ownership. That can produce impressive demos that never become trusted operating improvements.