Both Parse Server and Realm are popular choices for developing mobile apps. However, the choice between the two depends on your specific requirements and development needs.

Parse Server is an open-source backend-as-a-service (BaaS) platform that can be used to develop mobile and web applications. It provides various features such as user management, push notifications, cloud storage, and analytics. Parse Server can be hosted on any cloud service or on-premise infrastructure.

Realm, on the other hand, is a mobile database platform that provides real-time synchronization and offline capabilities. It can be used as a replacement for SQLite and Core Data and can be used on all major mobile platforms, including iOS, Android, and React Native.

If you require a fully-featured backend with features such as push notifications, user management, and cloud storage, then Parse Server may be a better choice. On the other hand, if you require a mobile database with real-time synchronization and offline capabilities, then Realm may be a better choice.

It's also worth noting that Realm can be used with Parse Server to provide real-time data synchronization between the client and server, which can be useful in certain use cases.

Ultimately, both Parse Server and Realm have their own strengths and weaknesses, and the choice between the two depends on your specific app requirements and development needs.