In my opinion, teams should choose Azure Functions over App Service when they are building event-driven, lightweight workloads that need automatic scaling and cost efficiency, such as background jobs, data processing tasks, scheduled automation, or microservices that respond to triggers like HTTP requests, queues, or timers, because Azure Functions follows a pay-per-execution model and removes the need to manage infrastructure. On the other hand, if the application is a full-featured web app, long-running service, or requires consistent performance, more control over the environment, and complex hosting needs, then Azure App Service is usually the better choice. The most important factors in this decision are workload pattern (event-driven vs always-on), scalability requirements, cost predictability, execution duration, and the level of control needed over the runtime environment. Overall, Functions are ideal for short, reactive workloads, while App Service is better for stable, continuously running applications.