I would say that it depends on your son's personality and preference, so I would talk to him and see which option he is interested in. You might also talk to the school as well and see how hard it will be to switch to 1/2 day if full is too much too fast. Kindergarten can be pretty intense as it is probably the first time your son will have to sit quietly for long periods of time and share with a large crowd.
I have a quiet and sensitive little boy, so we decided on a 1/2 day program and it works great for him. He gets to see his friends and be active in class for the morning. The half day class gets to go to the library, gym, recess and media center along with in class activities.Then he comes home and gets to relax and spend quiet time with his family. He loves school and comes home happy, but I noticed that he gets overwhelmed much faster on school days than weekends. His teacher thinks that he tries so hard to be perfect in school that he just overflows once he gets home. So I think a full day program would be too much for him emotionally even if he was happy while he was in class.
The all day program in his school is classroom from 8:40 - 11:40 and then they go to basically a daycare run by the school after lunch. Then they are released when school is released. The kids in the afternoon class attend the daycare first. To my understanding the room is run by a teacher and she does work with them on the basics, but it is not structured like a class and is run much more like a preschool for older kids. There is often an additional fee for this program as well.
I personally would rather my kids were home with me and I was able to read with them and help them myself than pay someone else to get to spend time with them.