Building your own team buys you the most control. The engineers learn your customers and your data model over months and years, and that accumulated context sits in the building. The cost is a long ramp-up and fixed costs: filling a senior role takes months, onboarding takes several more weeks, and the payroll carries on regardless of workload.
Full outsourcing means someone else is accountable for shipping: they staff the project, the provider manages the day-to-day work, and they absorb the delivery risk. This works well when the scope is reasonably clear and you have an available product owner. It works badly when there is no one to answer questions, because the provider cannot invent your business rules.
Staff augmentation is the middle option: you rent capacity but keep responsibility for delivery on your side. It is fast — a matching profile can start almost immediately — and the commitment ends when the work does. The condition is that your own leads have to have the bandwidth to manage them. Without strong internal leadership, aso services company you are paying hourly for uncoordinated work.
Most of the time, companies blend them. A common pattern holds architecture, product decisions and core domain code inside the company, while an outside vendor handles peaks, well-defined modules or platform work. The principle which is better flutter or react native simple enough: keep the parts that are hard to re-learn, and outsource the well-trodden work.
Three simple questions resolve most of these debates. First: is what you are building central to how software outsourcing works you make money, or internal plumbing? Second: how long will you need this capacity — one project or a permanent roadmap? Last: who will maintain it in two years? Answer these three honestly and the model is normally clear.