I think Amber is a mildly pleasant actress. When she was forced to cry onscreen in Glee, it wasn't great. I think she'll be great in the role.
As for Jennifer Hudson, I haven't seen her in The Color Purple, but she was right for the character, in my mind, for the film version of Dreamgirls. She was awful in SATC. I thought she was pretty good in the Secret Life of Bees. I can't actually think of any other film acting she's done, off the top of my head. Give her a song to sing and she's dynamite.
"Contentment, it seems, simply happens. It appears accompanied by no bravos and no tears."