I really like both, and think they both might have a chance to be really good. They are both great defenders, I think Cason is better than Black, but Black is probably capable of defending more positions.
Cason projects to be at least an average offensive player, who you have to at least guard. I personally think he could be even better than that. Black potentially is a guy who you can't play because his offense is so bad, although I think he'll figure it out.
Your continued bewilderment on people liking Cason is particularly amusing as you have admitted to only watching one game of his. I invite you to compare his NCAA performances vs anyone else in the draft not named Jordan Hawkins.