When evaluating different managed testing service providers, key metrics to consider include service level agreements (SLAs) for testing turnaround times, defect detection rates, test coverage across various platforms, scalability of resources, adherence to industry standards like ISO and CMMI, client satisfaction scores, and cost-effectiveness. These metrics help assess the provider's ability to deliver high-quality managed software testing services, meet project deadlines, ensure comprehensive testing across systems, maintain reliable performance, and offer competitive pricing. Evaluating these factors ensures selecting a provider that aligns with your project requirements and quality standards.