When comparing Redis vs MongoDB with oplog, the Slant community recommends Redis for most people. In the question“What are the best databases to push updates from the server database to the client in real time?” Redis is ranked 2nd while MongoDB with oplog is ranked 6th.
Ranked in these QuestionsQuestion Ranking
Pro Redis is written in ANSI C and therefore doesn't have a VM
Pro Good support
Redis has great docs, an active mailing list, and a github community.
Pro Redis uses the BSD license
This means that it is able to be used in a commercial product if one wants.
Pro There are numerous client libraries and frameworks to make redis more powerful
For Node.JS for example you can get something similar to what Parse.com was offering by using Node ORM.
Pro Great speed
MongoDB queries can be very fast because the data is usually all in one place and can easily be retrieved in a single lookup. But this is true only when the data is truly a document. When it's trying to emulate a relational model it starts to become really slow because it may have to perform many independent queries to retrieve a single document.
Pro Easy to scale
MongoDB has powerful sharding and scaling capabilities for when the data stored in the database gets so large that a single machine may not be able to store all of it. Sharding solves this problem through horizontal scaling. Mongo gives developers the ability to easily and painlessly add or remove as many machines as needed.
Pro Doesn't require a unified data structure
Mongo is very flexible in that it doesn't require a unified data structure across all objects. So it's rather easy to use.
Con Reported to lose or corrupt data
MongoDB is famously known for leaking and losing data over time.