Tried MongoDB - early euphoria - later dread. Tried MySQL - not bad at all. Found PostgreSQL - will never go back. So much support for this it should be your first choice. Simple local (free) installation, and one-click setup in Heroku - lots of options in terms of pricing/performance combinations.
As much as I love AWS EC, I prefer Heroku for apps like this. Heroku has grown up around Rails and Ruby, massive set of add-ons that are usually one-click setup, and I once had to perform an emergency app scale-up a that I completed in seconds from my mobile phone whilst riding the Bangkok subway. Doesn't get much easier than that.
Although there are more options out there now, some with better pricing, it was the first and most widely-available tool for sharing content. Our app is asset-heavy (images, videos, icons, etc.) and it makes it dead-easy to move stuff around. Still love it even if it's showing its age.
We prefer CircleCI because we care about testing our apps. We found it is better to invest the time writing rSPEC tests to ensure we don't insert any regression bugs with new branches. It's also nice to have a fully-automated deployment process from GitHub to Heroku.