Both these arrangements have the audience members sitting very close together. This togetherness allows laughter to pass immediately from one person to the other. You will even see audience members elbowing and slapping their immediate neighbor on the knee. By far the best seating arrangement for laughter is semi-circular. When public speaking audience members are seated on a curve they can look to their left or right and see the faces of each person in the row.
Laughter is contagious.
Many people will laugh just because they see others laughing. In a straight-row theater style, when an audience member looks left or right, all that she sees is the ear of the next person in the row. If that next person is not laughing, the other audience member is less likely to laugh. If you change the seating arrangement to semi-circular where each audience member can see everyone's face in the row, you will create a much higher likelihood that that person will see someone else laugh. As the speaker you will have a much higher chance of having your audience enjoying laughter because of this seating style.