Different jobs
A technical co-founder shares long-term ownership and risk. A fractional CTO brings senior judgement without permanent hiring. An agency supplies delivery capacity, usually after scope is clear.
The wrong choice is expensive when the venture still has unresolved product or market uncertainty.
A practical rule
If the problem and scope are still moving, start with product and technology leadership. If the scope is validated and the governance is clear, delivery capacity becomes easier to buy.
Use the model page to choose the smallest useful next step.