Whether to use the Unity 3d or Unreal Engine really depends on the developer. There are many factors that make both engines advantageous to beginners, but Unity 3D’s extensive asset store is truly more beneficial for them. However, the Unreal engine’s source code access is preferred by a lot of developers as well.

All of that said, it should be noted that both engines are always evolving and changing at a rapid pace. This can make it difficult to accurately say which one is better. If you are a beginner, then choosing Unity which is easier and has a less steeper learning curve, is the better choice.