My partners of the last 30 years own both mules and horses. I'd personally favor a well trained mule over a horse any day. Pound for pound, mules can carry a bit more, and are less inclined to spook and instigate a total wreck. A couple have left a dead horse in the wilds, but never a mule.
Most long string packers I've seen in the wilderness run a string of mules and ride a horse.
