The difference the magazines pointed out was the Mustang was easier to live with. They claimed it had nicer interior, more comfortable ride, and they liked the looks better. I believe that is what killed the F-body.
Build quality was something to be desired on the F-body too.
"The vehicle was criticized for bland styling, and some fans of the original GTO complained that it lacked nostalgic styling cues."
Speaking for myself, this is dead on.
I like the modern styling on my GTO, and at 150+ MPH, you arn't going to get a second look!! Can you say sleeper?