I'll go ahead and toss my two cents in, not that there is much new ground to cover.
I lived in Knoxville for 5 years, played both and have gone back and walked around each for a closer look multiple times after.
It is really interesting that two courses that are very different "products" can both result in greatness. And, yes, both are great.
Nick, I would use this approach regarding which to play - look at which would be tougher to access "off the street". Without arguing which is better, it is pretty easy to conclude that The Honors is harder to get to play (though they have always been spectacular at accomodating someone who just wanted to walk around and look at the course). This should weigh in on your decision. Holston Hills, from my understanding, has a pretty inviting policy regarding outside play, so you could probably get on there another time.
All of that aside, both have really great qualities; and don't let anyone fool you into thinking The Honors is only about the "experience", the golf course stands up on its own merit.
#5, 12, 15 and 18 really reveal Pete Dye's talent with all types of 4 pars. In fact, #7 is actually a really good hole, it just looks like it would be more at home at PGA West that in the Tennessee foothills. I still have not heard a reason why its lake edge has wooden bulkheading while the rest of the course has more natural looking native stone work. Toss in some great par 5's and a solid set of par 3's (14 and 16 are way beyond solid, though in my book) and you have a really great course. David Stone's presentation of the course only makes everything better.
Holston, like others have said, is a course that seems better taken as a whole rather than individually. Plenty of good holes come to mind, though, like 2, 3, 8 (a scary little par 3), 12, 13 and the funky 15.
The real end result is that you won't feel like you lost out either way.
Keith.