I'm afraid to say that I am not aware of a yardstick of the type you mention.
Intermediate course books tend to cover the same language points. However, this does not mean they are at the same level of difficulty. It is the approach to the language point that makes it easier or more difficult. Another thing that differs is skills work.
You could try out some activities from a course book with your classes and evaluate how well the students cope. This will help you make your choice.