Hire in-house for continuous ownership — definitions, cadence, and the judgement calls that need company context. Use a partner for bounded builds that need specialist depth you won't need permanently: a CPQ implementation, a CRM migration, a territory redesign. The two are complements, not substitutes.
The economics case for a partner is strongest below $25M ARR, where the ratio implies one head per ~$12M and the first hire is indivisible. It is also strongest where the work is a project with an end date — the failure mode is using a partner for what is really a standing function.
Every claim above comes from work LeanScale published. These are the sources.