Problem with xacro:if value="${model_type == A_model_type}"

I'm trying to set up a Xacro where one section is based on a Model type passed as an argument (the first line below would be an external argument). No matter what model_type is set to, model A, B and C get included. I can get this to work by having a separate variable set to true/false for each model, but it's ugly. The following always loads all three models. I'm using Indigo.