Is your project a simple MVP or a complex and multi-faceted solution?
How important is it to you to know in advance how much you are going to pay?
How much detail do you know about how the final product needs to look and work? Do you want to still be able to change or add requirements after the start of the project?
How important is it to you that your product reaches its end users as quickly as possible?
Are there immovable deadlines for the completion of the project?
How competent do you feel about managing the development of the project?