Pricing for smaller workloads is better on MongoDB Atlas right now. The DocumentDB performance pays of for super large collections and really high read/write workloads.
As a DevOps consultant; if Amazon is already setup as a vendor, I would just use DocumentDB. Setting up a vendor can be a major hassle and is not worth the saving of a few $$ per month. It's also much cheaper than spinning up and managing a EC2 instance with MongoDB installed on it since most of the operational knowledge can be deferred to AWS.