You can see how much root development occurs. You can even see where they cut it off. So, it was bigger than that. This plant, this root can grow up to about ten feet deep. Well, that depends largely on the size of your site.
Plants are more expensive, and thus are more suitable for small sites.
They have the advantage, in that they will flower earlier. Often if you get 'em from a reputable nursery you can get plants that will flower in one to two years. So, the rewards are quicker. Also, I would assume they'd be a good idea for a slope situation. Yes, they can be used for controlling erosion where that's a concern.