there is a pretty clear difference. Seger, at least early on, would have the Silver Bullet Band on half of his albums, and the other half would be with the Muscle Shoals Rhythm Section. Again, a different sound (most of the songs casual listeners know were with the Silver Bullet Band). Petty and the Heartbreakers, as libertycoug said, were IMO clearly a band. So while they all had dominant leaders, I think all can fairly be considered a band, and not just backing musicians.