I'm a fan of Python/Django, myself. The ecosystem is great: Django REST Framework is great, Kombu/Celery is great and easy for task queues, and it's super easy to deploy to Heroku (all things considered).

Honestly, I haven't used Ruby on Rails, but I recently found a new job and RoR experience would have opened up more doors to me than even Python/Django, which is pretty popular. If you don't already have experience with Python/Django that would make execution a lot easier for you, then consider RoR.