If you are talking ranch type stock horses I have found the terrain and size of the ranch dictates the style of horse.
Up here a lot of ranch horses were a draft cross of some sort in the past, and plenty of draft, QH, TB and Morgan crosses. Now the modern QH is popular, a cutting/cowhorse line crossed with old school ranch. Also a leggier horse is popular for the long miles trotted where it takes 10+ acres to run a pair.
When we lived in east TX I noticed a difference in style of horse used. They liked an old school foundation style breed horse or even the smaller style horses that are the norm in the cutting showpen. However there you could run one pair to the acre, not many big places as far as acreage, no need for a big strided horse to cover ground.