Jongo is an API to query in Java as in MongoDB Shell and uses Jackson to map documents to POJOs. This API is a tiny layer over the official mongodb-java-driver
ReactiveMongo is a Scala driver (mongodb-java-driver is not used) created by the play framework team to query MongoDB in a real reactive approach.
Both Jongo and ReactiveMongo projects have a play plugin :
Play-jongo : https://github.com/alexanderjarvis/play-jongo
Play-ReactiveMongo : https://github.com/ReactiveMongo/Play-ReactiveMongo
Well so, if you are looking for an easy way to query MongoDB you can use Jongo, if you are looking for a play-style reactive approach, you can use ReactiveMongo but IHMO you should test both and use the one you are more confortable with.