Fixed price model is an ideal pricing mechanism for projects with a clear scope, for example if you will like to create a sales business strategy for your product in a particular geography then it lends itself well to a Fixed price model. You are able to predict and budget with certainty on what will it cost to get this done. In this model, the requirements are so clear that periodic deliverables at agreed upon timelines (milestones) could be expected.
The T&M Model is an ideal model for engagement when requirements are fluid and your sales business objectives are in the definition stage. You will not like the service provider to price in all the risk of unknowns upfront so you are better off starting with T&M model to define your sales objectives, gradually moving to a fixed price model when there is clarity on the scope and outcomes.E.g. Search engine optimization and Social medial marketing for your business where it may be difficult to define hard outcomes early on but they are still important activities for your business.