I think my dad's book is a classic work by an 'old school' naturopath. By this I mean that the emphasis is on his personal observations and clinical experience(s). So in essence you are getting inductive wisdom.
By the time I was ready to write ER4YT I had accumulated a tremendous amount of additional material, much of which confirmed JLD's original observations, but some which would have not been as simple to observe. Thus with ER4YT you are getting not only inductive wisdom, but deductive information as well.
Aside from discrepancies, which reflect the 25 year gap between publications, their major congruence (and still the most important) is to separate people by blood type into high (animal) and low (plant) protein diets, and aerobic versus yoga/ relaxation type exercise prescriptions. These, if anything are more firmly established than ever.
Both books are effective at their own respective levels. I have a friend who refuses to install any Windows software in his computer. He hates upgrading, and also prefers the lack of complexity and simple learning curve. Others want the latest software, with the attendant complexities, but greater capabilities.