Right, if the child is too tall for any head restraint, a high back booster would be needed instead of the Harmony LBB for the center position.
All else equal and lacking any information on real world risk of serious or fatal injury comparisons, I generally do prefer models that are simpler and easier to use. Of course, all else is rarely equal. Measurements, limits, fabric choices, price and other factors are all important, too. Some may prefer more options or adjustments, of course, but it's so hard to get parents to ever glance at a label or manual that I feel most are best served with minimal adjustments that are easy to do. Advocates and technicians may be obvious exceptions, but these are a tiny percentage of consumers. As always, YMMV!
I'll go test the Diono in my car and let you guys know how it went. I am pretty good at installing, it took awhile but I can get a rock solid install with the seats we have. I'll need to really master this seat though, because I'll want it outboard when we aren't carpooling (my 3-year-old enjoys tormenting his older brother if they are next to each other). I read a ton about car seats and know more than the average person, but until I accidentally stumbled upon a thread here about Honda not allowing latch borrowing, I had no clue! Luckily we hardly ever had a seat in the middle. There's always more to learn and every time I come here, I learn a little more and try to help friends as well. Thanks everyone. 
