After riding on these conditions for only a short period of time, I would like to answer your question this way:
Don't focus on breed, but focus on heart. My Sam had to literally pull himself (with me on top) up a very steep embankment during one ride.
My riding partners, who were at the top watching, called down to me to give him the reins. I was riding in one continuous rein and dropped it and grabbed onto his mane.
They said they saw him looking at the embankment and choosing his footing and that he dug his front hooves in before lunging up. It took him three good heaves and were were up on top.
His actions were described to me as heart. He didn't panic. He didn't make unnecessary movements and he thought his actions out.
Now, maybe I am humanizing it a bit, but he did carry us through. I dropped the reins and closed my eyes.
Sam has performance breeding in him and I can tell he enjoys the challenging trails. I'm not sure if that has anything to do with him being a QH, his specific breeding, or just his spirit. I doubt his spirit is unique to quarter horses.
This spirit will be the measure to which I judge future trail riding horses.
I guess what I am trying to say is that since generalizing breeds can lead to disappointment, maybe what you should seek is a set of proven behaviors in a horse, regardless of his breeding.