I just/I've just

Which of these is more natural in speech? (the thing is said a few minutes after I fired the assistant)

1) I just fired one of my assistants.

2) I've just fired one of my assistants.

Which of these would be more correct/natural in writing?

1) The door opened and out walked the assistant Cal just fired.

2) The door opened and out walked the assistant Cal's (Cal has) just fired.