Darry has a very serious personality. This may be because he's been forced into the position of taking care of the family, but he seems to be unable to have fun anymore. Ponyboy says that every once in a while he might go skiing with his friends but mostly he just works hard all the time to keep the family together. He worries about Ponyboy and Soda and comes off as overbearing. One example of this is when he hits Ponyboy. Again, this may be a factor of all the pressure he feels.
Soda, on the other hand, seems to be very happy-go-lucky. He dropped out of school and is described by Ponyboy as someone who gets drunk on life. He is also a very caring person, trying to help Darry and Ponyboy deal with the stresses they feel. He is always joking about everything.
The one time the reader sees him feeling serious is when Sandy leaves to live with her grandmother and then he gets back his letter to her unopened. He clearly cares very deeply about the people that are important to him and is deeply hurt when they do not feel the same way.