I have both books--and really, I think it depends upon how much time one has to a) read b) work on training exercises and c) how much help you need from someone else to complete the exercises.
I found Click to Calm an easy read, and easier to implement some of the exercises (although my life's circumstances make REALLY working on some things difficult due to consistency of my available time to set up situations--reactive dog stuff). I found CU a great read--but overwhelming since I am not an agility person, etc. I don't have the time that I would like to have to really work some issues.
I think if I was recommending one over the other to the average dog owner, it would be Click to Calm.