Answer
Both can be appropriate. A fixed price suits a sufficiently defined scope of delivery with known data, interfaces, acceptance criteria and assumptions. It does not mean that every change requested later is automatically included. For research, unknown legacy software or iterative product development, billing on time and materials can be more honest, provided that budget limits, priorities and regular results are agreed.
A combination often makes sense: a limited check or first iteration with a clear effort limit, followed by a fixed price for a better-understood scope. Operation, support and further development are shown separately. No contract model eliminates business uncertainty. What matters is transparency, small verifiable deliveries and a binding approach to changes.
As of 3 September 2026
Further reading
Related topic: Our approach