Ask HN: Flask or Django for Building RESTful API?

We are planning to build REST API for our next project. The project will be in the financial domain and we need to do lots of statistical analysis using Pandas, Numpy and Scikit-Learn and provide the results as JSON, which will be consumed by AngularJS app in the frontend.

Another thing will be the database backend, which we have decided to use MongoDB. Everything will be hosted on AWS.

The incoming vintage data is from Amazon S3, which is less in early stage but it has potential to grow folds very soon. So in future we may need to integrate Apache Spark as the backbone for processing the statistical computing and returning the returns as JSON.

So, which framework will be best to work with for creating REST API which can integrate well with MongoDB and in future Apache Spark? Please provide your insights on this. Thanks