I would say that the answer to this is that their hair is their pride and joy.
We know this for sure about Ponyboy. When he has to cut and dye his hair (when he and Johnny are hiding out in the church) he cannot stand the idea of what he is about to do. He states, straight out, that his hair is his pride.
For Soda, we aren't told quite so explicitly, but we are told in Chapter 1 that his hair is really quite nice, so it makes sense that it would be his pride too. Also, we know that all of the Greasers are into their hair because it is what sets them apart.
I have to agree with the first editor that the pride and joy for the two boys was their hair. The book "The Outsiders" mentions hair frequently. Pony Boy describes Soda Pop's hair on page 8 saying it was a dark golden blond like a shnny wheat gold when the sun bleaches it. He wears it long silky and straight. Pony Boy's hair is also blond like Soda's. When the Socs want to taunt the greasers they state they are going to cut their hair.
"Need a haircut greaser?"(17)
When one of them has messy hair, the other directs him to comb it. Pony Boy also styles his hair like Soda Pop.
"You know you look an awful lot like Soda Pop the way you 've got your hair and everything."(64)
Hair for the greasers was part of their identity. They did not own nice cars or have nice things, but their hair was the one thing that they could control.
The pride of Ponyboy and Soda much be their looks. Ponyboy loved his hair and had to go through the hard time of cutting it. While Soda just loved the way he looks.