At the beginning of Chapter 3, Pony says that the two types of people are essentially the same, down deep. But all he really talks about is how they are different. He and Cherry decide that they are different because the Greasers let their real selves show and the Socs keep things bottled up. But we're not really told why they're similar.
To me, what makes them similar is that they are both groups of teens who are somehow trying to figure out a way to deal with what society thinks of them and expects of them. The Socs have to deal with the expectations that come with being rich. The Greasers have to deal with people thinking they are thugs. But both groups are really trying to figure out how to be themselves.
I think that's a lot of what this book is about -- teens trying to be themselves while dealing with what society thinks they are and what society tries to push them to be. Both the Greasers and the Socs have to deal with that.
In the book "The Outsiders" the similarities between the Socs and the Greasers are not often seen, but are very real. Both groups participate in violence in the form of fights and threats. The Socs as well as the Greasers have the dynamics of family problems. They are also all teens and young adults trying to be connected to something bigger than them. They need the people within the independent social groups to help them to feel strong and accepted. They have dreams about their futures but seem trapped in their tumultuous present. There also appears to be some degree of jealousy from one group to the next.