(SSE-SQS) uses SQS’s own built-in encryption and doesn’t involve KMS, so it’s free, faster, and avoids KMS throttling—but offers no audit logs or key policy control. KMS_MANAGED uses an AWS-managed KMS key (aws/sqs), which adds KMS logging, rotation, and policy features, but also adds cost, latency, and KMS API limits. Both are secure; the difference is performance vs. KMS governance features.