I think Nolan put it well. Ra's is more of a religious leader. Bane is more a militaristic leader and conqueror, but coming from the same school of thought or "religion" as Ra's.
Ra's has the refinement and dignity to get close to someone like Bruce Wayne and have him convinced that he's this great, righteous dude. Bane, while strangely having his own eloquence and wisdom, kind of perverts these traits by virtue of being this hulking mass with a mask that looks like a muzzle. He really looks and feels like a caged animal. It just embodies an entirely different archetype. Ra's was kind of a gentleman's bad guy. Bane is a beast forged from hell. Ra's tried to tame him, and I think that's where a lot of Bane's sophistication comes from, but ultimately I think Ra's was right...he's a monster. He's a monster with human feelings of course, but a monster nonetheless.
Ra's was the type of guy who would walk up behind someone who stood in the way of "true justice" and stab them in the heart. Bane is the type of guy who who will walk up right to the face of anyone who stood in his way, grab them by the throat and snap their neck like a twig, just to set an example.
Likewise, Ra's sought to use a chemical agent to make Gotham destroy itself, and then vanish into the night. Almost like an evil sorcerer or magician (I believe Nolan also said he likes to think of Ra's as a magician), or of course, a ninja- which he was. Bane on the other hand, took the League of Shadows...out of the shadows. Their approaches couldn't be more different in that regard. Ra's would have never been someone to rule over Gotham like a dictator. Bane was all about demonstration of power, which I believe is born out of that desire to prove himself the superior man to Bruce and Ra's. He wants you to see what he's going to do and how easy it's going to be. He wants you to squirm in the inevitability of defeat and death.
"I will show you where I have made my home whilst preparing to bring justice....then I will break you."
"When Gotham is ashes...then you have my permission to die."
"Search him...then I will kill you.
Bane is the once oppressed, who became the oppressor. He can speak for the oppressed in a deceivingly authentic way, given his background and how much he has suffered in life. This is what gives him that power of persuasion and makes him revered to the point where his own men gladly are willing to die for him (I never got that impression with Ra's). But ultimately, he is an oppressor of the most severe kind. Ra's was a lot of things, but he wasn't that.