Stephen Colbert is much edgier than David Letterman was. He is still a straight white man, but he is more liberal and edgier. His brand of humor is also differnt because he uses satire as his main for of humor, rather than being straightforward. He is also not afraid to dig deep at his subjects.
David Letterman dabbled in politics every once in a while when he felt he had something to say, but it was pretty rare. Stephen Colbert, however, makes politics his platform of comedy which makes it fairly edgier just for the simple fact that politics are edgier. Politics are really divided in this country right now and so if you take a stand, you are bound to insult a large population.
Stephen Colbert is not edgier than David Letterman was. Colbert is certainly not funnier, and his schtick gets old very quickly. It is a repetitive method that he uses to tell the same jokes, using the same delivery, over and over and over. Letterman was genius. Colbert simply attacks the right. That's just not funny after about ten minutes.
No, Stephen Colbert is not edgier than David Letterman was. Not only can Letterman be seen as objectively edgier by virtue of his material. He can also been seen as edgier due to the facts that he performed on network television and that he was active during an earlier era.