So, which one of the pricing models is right for your business? It depends on several factors, and the size of a software project is surely not one of them. You may address a Drupal development services company to build a business website (which, logically, should be a Fixed Price contract) and end up making a hundred change requests related to UI/UX design. There are seemingly complex projects which involve IoT technologies or integrations with new or close source APIs; yet, a customer has a tech background and a SRS up his sleeve, so going FP still makes sense. Don’t jump to conclusions; address a reliable vendor and take the time to analyze project requirements instead! Also, take a look at this article https://steelkiwi.com/blog/comparing-3-popular-pricing-models-fixed-price-time-materials-and-milestone/
Your pricing model will depend on your qualifications, like what you are requiring to be done, the working hours etc. The question is what gives you trust to an unknown outsourcing team that you will hire, without wasting your time and prepayment amount. I suggest, to try both approach for it can help you to determine what is the best for your needs and satisfaction.