You mean when Obi Wan defeated Maul vs. when Anakin failed to defeat Obi wan?
I think it comes down to the fact that in both occasions Obi Wan's mind was at peace. As Yoda said the light side is stronger than the dark side. The dark side is only quicker, easier, and more seductive.
I kind of wonder if the duel in Episode 4 could have turned out differently. As Darth Vader told Tarkin, to escape was not Kenobi's plan. I think from the moment the tractor beam pulled the Falcon in, Obi Wan fully intended to die on the Death Star in order to be a Jedi ghost and guide Luke.
I like to think that Darth Vader was holding back a little so he could enjoy the kill. Perhaps Obi Wan was too.
I guess it doesn't matter though. It ends the way it ends and that's that.