A good way to understand this is to read the Seam ref doc or a Seam book and then search the Seam core source or Seam example source code for examples of when/how to use those two annotations.
@Out is a direct outjection to a specified (or default) scope whereas @Unwrap is associated with the manager component pattern.
The @Unwrap method is called every time the component's name is referenced in a JSF, for example. Outjection occurs for all @Out annotated context variables after a business method successfully completes.
The concern is not performance, they are used for different scenarios...