In visionOS, I want show a 3D Content, I can use RealityView or Mode3D, But the effect they achieve is similar. What is the difference between them and which one to use for users?
The effect is similar because the two types are closely. RealityView being a top-level view that renders 3D content (using the RealityKit renderer) and provides RealityViewContent as "context" - a means to manipulate any of the entities, adding, removing, updating them, etc. If you were to use RealityView by itself, you're responsible for loading models, updating the situating the entities so they work as you expect, etc.
ModelView, by comparison, reads as those it's meant to the "fast path" to getting a model loaded and visible, without providing all the open context to manipulate the RealityKit renderer and entities. Model3D isn't exposing the context and doesn't provide a path to adjust, manipulate, and so on - any of the models you're loading and displaying.
The article Adding 3D content to your app covers both of them with snippets that might make it easier to see in context.
