Upon watching the light saber duel between Obi-Wan Kenobi and Darth Vader on the Death Star, and later pondering it, I formed the opinion that that duel was as intense as their earlier duel on Mustafar. Just as intense, but on a completely different level.
The duel on Mustafar was between two relatively young Force-users. Newly appointed Darth Vader had only recently embraced his fear, anger, and hatred. Obi-Wan, while a Jedi Master is every sense, had only been trained in the use of the Force for about thirty or so years. Both the Jedi and the Sith were still at the point where their physical actions defined them and provided them with security and comfort in a difficult situation.
Jump ahead 20+ years and you have tow vastly different individuals.
Darth Vader has existed in a artificial body with anger and hatred being the main supports of his life. As the Empire gained more power and authority throughout the galaxy, the opportunities for Vader to utilize his full physical skills became rarer and rarer. He probably relied on a quick Force choke for individual punishment, and a legion of Stormtroopers for groups. The light saber at his belt probably was more for intimidation of others; and a sense of confidence for Vader himself. It is doubtful that Vader had even had a use for it for the last 10 years or so.
Obi-Wan Kenobi spent the 20+ years as a hermit on a barren planet. His only connection to anything physical was keeping himself alive in the desert and being available to look after Luke from a distance. In learning from Qui-Gon Jinn about becoming one with the Force after death, Obi-Wan looked inside himself and his connection to the Force. His light saber was probably put away and barely touched, if at all, during the last 10 years or so.
Obi-Wan rejoins the galaxy at-large. He now has two missions: get the plans to Alderaan and show Luke the way of a Jedi. Deep in his heart, he knows he doesn't need to stay alive for either mission. Once he realizes that it is up to him to shut down the tractor beam, he has no intention of leaving the Death Star alive. The years of meditation and training for becoming one with the Force, while maintaining his sense of self, was now coming to fruition.
Darth Vader for many long years had no real adversary. His presence alone caused other beings to do what he wanted them to do. Where once Anakin Skywalker lived for the thrill of action, Darth Vader used his connection to the dark side of the Force to fuel his thoughts and his life. Since he never encountered anyone as strong as him as an adversary, he didn't need to practice or even think about his physical skills.
Now these two Force-users, one Jedi and one a Sith, meet again. Vader's anger from a betrayal and fear for a loved one has morphed into a desire to maintain power; whereas once the Force (and his use of it) served a higher purpose, now the Force served Vader himself----he used it to maintain his life.
Obi-Wan, knowing that his death will bring an opportunity to support and assist Luke in ways he never could while alive, is preparing for that death. Rather then looking for a way to safeguard the galaxy with his skills and focus, Obi-Wan is looking inside himself and his connection to the Force.
When watching the duel on the Death Star, I get the feeling that both Vader and Obi-Wan use their light sabers as a focal point of the conflict. But the real intensity is going on within the Force, and within their hearts and minds.
But not surprising, Obi-Wan has a greater insight into what they have both become. He also acknowledges that both of their "powers" reside more on attitude and thoughts than light saber dueling. The verbal exchange reflects this:
VADER: When I left you, I was but the learner; now I am the master.
KENOBI: Only a master of evil, Darth.
In this simple response, Obi-Wan is both pointing out that Vader is a master of evil. Evil is a state of behavior. It doesn't automatically reflect skill or power---just attitude. Obi-Wan also calls him "Darth" which I think is patronizing and an acknowledgment that all that is left of Anakin Skywalker is a Sith Lord and nothing else. After defeating Vader on Mustafar, Obi-Wan vents is sadness and frustration with his former Padawan. But on the Death Star, Obi-Wan only sees the evil.
VADER: Your powers are weak, old man.
KENOBI: You can't win, Darth. If you strike me down, I shall become more powerful than you can possibly imagine.
In his fear, Vader gravitates back to his reliance on action. He considers Obi-Wan's physical powers and age, and declares them both inferior to Vader's. Obi-Wan however, knows that the battle he is fighting is not on that deck, but in the heart and mind of Luke. Obi-Wan knows that Vader can't win, since Vader will only rely on his physical skills, while Obi-Wan is operating on a completely different level.
While the minimal light saber exchanges are happening, the real battle is going on underneath the physical movements. And it is a battle that Vader completely loses to his old Master. And by winning that battle, Obi-Wan takes Luke on his first steps to becoming the first of the new Jedi.