If an app is slow, not user-friendly or is taking too long to load the information, then the customers don't wish to use it anymore. And they start looking for better alternatives. Any app has to be near to perfect to increase a company's business in the mobile world.
Following is the list of few factors to be considered while selecting the best provider:
1) Complete Coverage of Testing Services: The service provider should have complete testing coverage. To verify if all the functionality is completely covered, you can have them provide some sample test cases or suites for some given functionality. That way by looking at the sample you can judge how good the coverage is.
2) The Number of Mobile Testing Projects Delivered Successfully: Whenever you are taking testing services from any provider, make sure that you ask them to provide the details of mobile projects that they completed successfully. It can be details like feedbacks, reports, contact details of their clients, etc.
3) Mobile App Test Lab and Devices: Take account of the test labs and take data about the number of devices, the number of devices with your required OS version, etc to check if their labs are matching your testing requirement.
4) The Number of Mobile App Testing Specialists: The service provider should have sufficient testers for different testing that needs to be done for the app. There should be specialized testers for manual, automation, performance testing, etc. Apart from this, there should be some extra members in case of emergency or testers exiting.