Fantasia's vocals are, by themselves, too uncontrolled for her to play Mahalia convincingly, since Mahalia's vocals were always very restrained and controlled (but still v. powerful).
Will be interesting to see if she can tone it down for the movie.

Other than that, Mahalia was a GOSPEL singer. Why label it soul when it wasn't, she never sung soul or R&B or anything mainstream, it was always plain gospel.