T'Challa is a king with Genius Level intellect....I don't see anything about MJW (as much as I DO like him) that tells me he can do all that. People are just thinking action....and we know all of these Marvel movies go just as much into the characters and their lives as they do into the action. I don't see that layer from Jai White. I could buy him more as Cage, personally.
Yea, I don't know how tall MJW is (Luke is typically drawn as towering over anyone not Thor/Hulk/Thing/etc) but as long as he ain't some DMX type midget standing next to everyone else, playing Cage would be the better fit for him imo.
Also, T'Challa is a genius level intellect who gets into verbal spats with Tony Stark on some "anything you can do" s-hit, so you gotta have an actor that can have a witty side and get at people like Downey Jr.'s (type of) character.