When deciding between EC2 and serverless primitives in AWS, consider the following factors: Use EC2 when you need fine-grained control, have legacy applications or specialized software, or deal with long-running, predictable workloads. Opt for serverless primitives, like AWS Lambda, for event-driven, short-lived workloads that benefit from automatic scaling and pay-per-execution pricing. Consider performance, cost, security, and your development team's familiarity with each option.