What the best option may be for you can depend on several factors:
- How often are you going to poll the API?
- How frequent are you going to be inserting into the DB?
- How often are you going to read from the DB?
- How fast will the data grow?
- What is the structure of the actual data?
MongoDB horizontal scales very well meaning that if you are going to be polling the API frequently and inserting large volumes of data into your DB then it is relative easy to implement sharding and fire up another Mongo instance on another server. Difference between scaling horizontally and vertically for databases
Inserting JSON data into MongoDB would probably be simpler than converting it to a relational format and maybe doing several inserts where (assuming the JSON is formatted properly) you could just do a single insert. Or use mongoimport to import many JSON documents at once. If the API data changes quite often for instance new columns, then in a relational database you will have to create the columns using ALTER TABLE, whilst in MongoDB it will just store the JSON document.
If you want to know more info about MongoDB they do offer free online courses. The courses are 7 weeks long , Each week they release new video lectures and a new assignments which take a couple of hours to get through. https://university.mongodb.com/courses/catalog