<<Hogan writes for the pros and Darrel writes for the enthusiasts, IMO.>>
Mmmm, think that's a bit unfair. I'm no pro, but for me as a new reader of Thom, he made my D800 come to life. That's not to say I wasn't already reasonably competent, but I just wish I had used his D7000 book and avoided a nightmare journey.
Darrell, as you say, writes in a more relaxed way and produces a very comprehensive book which is always well written.
Basically, I believe that the mass majority written by both would already be understood by most readers except a novice. It's the subtle changes of a new model that may require refinement to our shooting techniques and that's what most of us want I think from either author.