Believe it or not, I actually got through almost all the posts on this thread. I skipped the last couple pages because it was just the same info over and over.
Thought I'd at my 2 cents worth.
Almost all the threads have been about horses, because the opening thread was asking what is the best trail breed. Everybody has their opinion based on their own experience. So, the thing that needs to be asked is this: What kind of trail riding do you intend to do with the horse?
A lot of the talk has been about speed. My uncle was an USFS worker who guided hunts in southern Utah and did a lot of back country riding in his spare time. Once a bunch of my nephews and nieces visited him and were out riding his horses. When he came home from work and they told him how much fun they had cantering up and down the road, he exclaimed, "I'm surprised they didn't get bucked off! Those horses don't even know they can canter!" In other words, he had no use for anything other than a walk, and occasionally a trot, on the trails he rode. I find that I train my horses to canter and change leads just as a training matter. I never go above a trot out on a trail. Most of the trails I have ridden in my life would not allow anything above a slow jog safely. On the other hand, I can't stand a horse that won't keep up a steady walk.
If all your riding is on developed and maintained trails, I would suggest you go with the breed your riding partners and friends have. That will save you a lot of frustration as far as matching gaits.
If most of your riding is in the mountains on poorly maintained rough trails, having a horse that paces or maintains an 8-9 mph run-walk will find little use for that gait.
I have used 1/2 Arab/QHs, QHs, mustangs, and other grade horses (mostly QH type). I have enjoyed them all and they have all done well in the mountains.
As for steady and calm, I have to say my 1/2 Arab/QH stallion was the best. He was only about 14 hands at age 2 when I started training him. He had the smoothest jog of any horse I ever rode. We were jogging along one day with the family, on their tall QHs, when Char (my Arab) put his nose down to sniff the trail as he jogged along. He suddenly stopped dead in his tracks and I nearly went over the front. When he wouldn't go on, I noticed his ears were working back and forth like he was confused. He wouldn't lift his head. When I got off to take a look, I found he had stepped through his rawhide bosal with his front left. He looked at me like, "Well, help me out of this!", but stood quietly. I lifted his hoof out of the bosal and we kept on going. What a wreck that might have been with another horse.
I had a registered QH mare with excellent "Bar" breeding that would outwalk any other horse I ever rode or rode with, for distance and speed. She stood a little over 15 hands. She once brought my dad and me out of the mountains on a dangerous trail when it was so dark we could not see each other. I wouldn't have sold her for a million bucks that night. Later on in her life, as she approached 17-18 years old, she began to suffer from cramps and would "bind-up". When that happened she would literally explode and go to pieces in a blind panic. That became so dangerous I had to quit taking her on rides. Her son, also a registered QH with great breeding, developed the same malady as he approached 16 years. Binding-up has a technical name, but I forget it right now, but it is a malady that has shown up in QHs from time to time.
My Dad has a QH with championship cutter breeding that stands about 14 hands or maybe less. He will go anywhere you want to point him (so don't point him places you are not sure you want to go!) and he can walk as fast as any horse I know. He will keep up with and even outwalk 16-hand horses on the trail. He is one of the most personable horses I have known.
I had a mustang that was great on the trail. Never, and I mean never, got hurt. He could live in a field of barbed wire and never get a scratch. Stood about 14.2 or so. My only complaint with him was that heading east he walked slow and I would have to keep after him to keep up a good pace. Walking west, he would walk fast. Didn't matter where we were. All I can figure was that he was raised on the Navajo Reservation, or somewhere west of New Mexico and he was going home when he headed west.
As I get older, I don't think much about speed, except that I like a fast walk. I have found that I don't like registered horses, no matter the breed. Same with dogs. It seems to me that registered horses and dogs have cost me way too much time and money in vet bills. Grade horses and dogs seem to be heartier and healthier. For me, when I go looking for a horse, I look for good conformation, healthy hooves, a soft eye, and good personality. I also try to assess whether the horse has ever had a serious injury. Since registered breeds are often used in competitive sports, such as jumping, roping, barrels, etc, you often see folks selling them after a serious injury that precludes them from continuing competition. You seldom see that kind of thing with grade horses. I could care less about breeding as long as the body-type is what I'm looking for. I just want a horse that will be willing to go and will carry me or my loved ones safely through rough country.
Attached is a picture of my daughter on a 7 year-old grade mare I am training for her in VA. The horse with her is a friend's registered QH at age 20. The other pic is the little QH cutter I mentioned in the White Mountains of AZ.