Direct answer
Use fixed-price (or fixed-scope milestones) when the outcome is clear enough to write down. Use time-and-materials or a retainer when priorities will shift weekly. For most first products, we lock a scoped v1 after discovery rather than open-ended hourly drift.
The best model is the one that matches uncertainty. Clear v1 → scoped price. Unknown discovery → workshop or T&M first, then a fixed build.