It's tough to recommend one over the other. The authors have very different style.
Smith is much less technical/mathematical and tends to speak from his wealth of experience. Tune to Win is a great book for someone who may not be an engineer, but still wants to learn a bit about vehicle dynamics and car setup.
Race Car Vehicle Dynamics by Milliken is the bible of, well, race car vehicle dynamics. It's much more technical, and is presented with an engineering audience in mind. That said, it is much more thorough and comprehensive. I like it since it presents the fundamental dynamics of the various vehicle systems and builds the analysis from there to arrive at meaningfull conclusions.
So, it really depends what you want to get out of the read. Yup, a typical "It depends" answer from an engineer.