The way I'd approach this is to carry out a survey. Prioritise a list of important criteria, such as performance, functionality, and cost. For example with MongoDB you can archive documents if the data not immediately required to save on costs at the expense of instant access, but if that fits your use case model then you can use that feature. So create a use case test project that actually uses both services as per your use case and see for yourself the results of the tests. Along the way you'll encounter issues perculiar to each platform that you can factor into your final decision, such as comparing how easy it is to use their API, or that the documentation is sparce or confusing. From there you'll have an informed decision and you'll be confident investing further resources into it.