Songsam and Tokchae are much more alike than they are different. They were childhood friends before the civil war began ripping apart families and communities. The initial separation of Tokchae and Songsam probably indicates the one main difference that existed between the two men. Tokchae chose to stay in his village with his family and fight. Songsam, on the other hand, ran away and went into hiding. He left his family behind. That single incident illustrates a few differences. The case could be made that Tokchae is braver and more family focused than Songsam.
As for similarities, there is likely an entire list of similarities that I could make up. Being childhood friends would indicate that they like the same kind of stuff (food, sports, girls, hobbies, etc.). It doesn't matter what time period or location they are from. They are friends for a reason, and that's likely because they are similar is a lot of ways. The story doesn't go into any of that though. What is clearly indicated is that they both value freedom. Songsam ran away in order to maintain his freedom. Tokchae stayed behind in order to have the freedom to care for his family. The flashback sequence in which the boys free a crane is symbolic of that similarity.
But the next moment, as another crane from a nearby bush fluttered its wings, the boys' crane stretched its long neck with a whoop and disappeared into the sky. For a long time the two boys could not take their eyes away from the blue sky into which their crane had soared.
That love for freedom is not simply a distant memory of the men's past. It is still active in their hearts. It's why Songsam unties Tokchae and gives him his freedom.