There's one thing I don't get in ArangoDB:
What's the difference between an edge collection and a graph? In which cases should I choose which?
Graphs in ArangoDB are built on top of documents and edges.
Edge collections have automatic indexes on _from and _to, allowing efficient retrieval of any connected documents. Because data are still stored in regular (document and edge) collections, you can use these collections in non-graph queries, too.
Graphs add some functionality (i.e. query methods, traversals) on top of the data. You can have multiple of them in ArangoDB. Think of a "graph" being a means of grouping for parts or all of your data, and making them accessible in queries.