What did David and Laurie (argue) about?

They argued about the wave being beneficial or not. Laurie thought it was more of a cult than a positive student movement for social change. 

What about Brad and What about this answer ""Laurie tells David that her mother is worried that The Wave is some kind of "brainwashing" and that Ben Ross "is manipulating" the students. David says Laurie's mom is nuts and that all she does is worry about everything all the time. (Hey, our moms do that, too! Maybe it's a mom thing.) A little disagreement starts to erupt. Laurie gets defensive: she never said she agreed with her mom. But David points out that she didn't say her mother was wrong either. Laurie wants to continue this discussion with David about The Wave, but she doesn't want them to argue, so she backs down"" .